BOWMAN, Circuit Judge.

Defendant Ben M. Hogan Company, Inc. (Hogan), a corporation engaged in the business of highway construction, was indicted by a grand jury on one count of conspiring to restrain trade in violation of the Sherman Act, 15 U.S.C. § 1, and on three associated counts of mail fraud under 18 U.S.C. § 1341. Hogan was found guilty on all counts charged in the indictment. The District Court fined Hogan $800,000 on the Sherman Act count (Count One) and $1,000 each on the mail fraud counts (Counts Two through Four). Hogan appealed. We reverse Hogan's conviction on Count One and remand this case to the District Court for a new trial on Count One. We affirm Hogan's convictions on Counts Two through Four.

The theory upon which Hogan was prosecuted was, in essence, that between May 1979 and September 1980, Hogan conspired with A.P.T. Construction Company, Inc. (A.P.T.) to allocate between themselves, through the submission of rigged bids, work contracts for certain state highway construction projects. The government's principal witness, Leonard Thompson, president of A.P.T., testified that at the request of William Moore, vice president of Hogan, he agreed in May 1979 to submit a complementary bid, i.e., a higher bid than the bid Hogan planned to submit, on project 60173. Two bids were submitted on project 60173, one from Hogan and one from A.P.T. Hogan's bid was the lower and Hogan was awarded the contract. According to Thompson, in exchange for A.P.T.'s complementary bid on project 60173, Moore promised to return the favor in the future.

Thompson testified that in July 1980, he telephoned Moore with a request that Hogan bid in excess of the amount A.P.T. planned to bid on project 60179, and that Moore agreed to do so. Thompson also testified that he was able to make a similar arrangement regarding project 60179 with Jack Freshour, president of Freshour Construction Company (Freshour Construction). A.P.T., Hogan, and Freshour Construction bid on project 60179. The contract on project 60179 was awarded to A.P.T., which proved to be the low bidder.

Thompson further testified that in September 1980, he telephoned Moore with a request that Hogan bid in excess of the bid A.P.T. planned to submit on project 60234 and that Moore agreed to do so. Thompson testified that when he made the same request of Freshour, Freshour told him that Freshour Construction did not intend to bid on project 60234. Hogan and A.P.T. were the only bidders on project 60234. A.P.T. was the low bidder and was awarded the contract on project 60234.

Thompson's testimony was contradicted by several witnesses. Moore acknowledged that he had had conversations with Thompson concerning projects 60173 and 60234 but denied entering into any agreements with Thompson to rig bids. Freshour testified that he never discussed making complementary bids with Thompson or anyone else. Freshour's wife testified that she had never encountered Thompson, while Thompson's testimony suggests that she had been present on one occasion when he claims to have met with Freshour at the Freshour residence.

Hogan's bid estimator and its asphalt plant superintendent testified that they never had been instructed to keep a bid above any particular amount, and that, in fact, they had standing instructions to update bids with price reductions if a downward price fluctuation during the pendency of a bid made it feasible to do so. Freshour Construction's bid estimator testified that he cut the profit margin on his company's bid on project 60179 in an effort to secure the contract.

Others eligible to bid on the projects in question testified that they were not contacted regarding the bidding on the projects alleged by Thompson to have been rigged. Thompson, however, testified that it was unnecessary to contact other bid proposal holders because their plants were located so far from the project sites that it would have been impossible from the standpoint of transportation costs for them to have underbid.

For reversal, Hogan argues that the District Court improperly instructed the jury regarding the interstate commerce element of the Sherman Act violation charged in Count One of the indictment. We agree, and we therefore reverse Hogan's conviction on Count One and remand this case to the District Court for a new trial on Count One.

Hogan also makes a number of other claims, all of which we have carefully considered and have found to be without merit. We will, however, discuss Hogan's additional claims to the extent that they are not resolved by our reversal for instructional error on Count One.

I.

The District Court correctly characterized this case as one concerning acts which, if proved, are deemed per se unreasonable under the Sherman Act. Having done so, the court instructed the jury that:

Certain types of conduct are regarded as unreasonable per se. This means that the mere doing of the act itself constitutes an unreasonable restraint on interstate commerce, and it is not necessary to consider why the acts were committed, or their effect on the industry, or any other explanatory matter. Conduct regarded as unreasonable per se includes price fixing, division of markets and bid rigging.

Trial Transcript (Tr.) at 492 [hereinafter referred to as per se instruction] (emphasis added). Hogan argues (1) that the per se instruction informed the jury that if it found Hogan's actions constituted bid rigging, it could convict Hogan without considering whether the government had proved that Hogan's actions had an effect on interstate commerce; (2) that the per se instruction created a conclusive presumption of an effect on interstate commerce; and (3) that the government thereby was unconstitutionally relieved of its burden of proving every element of its Sherman Act case.

"A conclusive presumption removes the presumed element from the case once the [government] has proven the predicate facts giving rise to the presumption." Francis v. Franklin, ___ U.S. ___, 105 S.Ct. 1965, 1971 n. 2, 85 L.Ed.2d 344 (1985). The Supreme Court has found conclusive presumptions in criminal instructions to "`conflict with the overriding presumption of innocence with which the law endows the accused and which extends to every element of the crime,' and [to] `invade [the] factfinding function' which in a criminal case the law assigns solely to the jury." Sandstrom v. Montana, 442 U.S. 510, 523, 99 S.Ct. 2450, 2459, 61 L.Ed.2d 39 (1979) (quoting Morissette v. United States, 342 U.S. 246, 275, 72 S.Ct. 240, 256, 96 L.Ed. 288 (1952) and United States v. United States Gypsum Co., 438 U.S. 422, 446, 98 S.Ct. 2864, 2878, 57 L.Ed.2d 854 (1978)).

We have examined the per se instruction and we conclude that it could have been understood by the jury to include an unconstitutional conclusive presumption of an effect on interstate commerce. We are convinced that the District Court did not intend to create such a situation; immediately prior to giving its per se instruction, the District Court gave a correct instruction on the interstate commerce element and on the government's burden of proof regarding that element. The per se instruction, however, contradicts the court's careful instruction on interstate commerce as a separate element of the Sherman Act offense. "Language that merely contradicts and does not explain a constitutionally infirm instruction will not suffice to absolve the infirmity. A reviewing court has no way of knowing which of the two irreconcilable instructions the jurors applied in reaching [a] verdict." 105 S.Ct. at 1975.

If ever there could be a case where the giving of a jury instruction that included a conclusive presumption concerning an element of a crime amounted to harmless error, this would be the case, for in the record before us there appears to be ample evidence from which the jury could have found — and perhaps did find — that the element was proved. But where "[t]he conclusive presumption the jury was instructed to apply permit[s] the jury to convict [the defendant] without ever examining the evidence concerning an element of the crime charged," a plurality of the Supreme Court has held that "[s]uch an error deprive[s the defendant] of `constitutional rights so basic to a fair trial that their infraction can never be treated as harmless error.'" Connecticut v. Johnson, 460 U.S. 73, 87-88, 103 S.Ct. 969, 977-978, 74 L.Ed.2d 823 (1983) (quoting Chapman v. California, 386 U.S. 18, 23, 87 S.Ct. 824, 827, 17 L.Ed.2d 705 (1967)). While the Connecticut v. Johnson plurality did not hold that "instructional error of constitutional dimensions may never be harmless," it did counsel that not every "form of instructional error should be analyzed for harmlessness." Id. 460 U.S. at 83, 103 S.Ct. at 976. Though the substantive issue of the flawed instruction in Connecticut v. Johnson was the critical issue of criminal intent, rather than the issue of an interstate effect on commerce, the form of the instruction clearly was that of a conclusive presumption. And the plurality's summary of its holding, see id. at 87-88, 103 S.Ct. at 977-978, indicates that it was the conclusive form of the instruction, more than the particular element of the crime to which the instruction pertained, that was of concern to the plurality.

Accordingly, we hold that by including the word "interstate" before the word "commerce" in its per se instruction, the District Court committed reversible error. We therefore reverse Hogan's conviction on Count One, the only count to which the per se instruction related, and remand this case for a new trial on Count One.

II.

Hogan claims that it is entitled to reversal of its convictions on all counts of the indictment due to the government's failure to make available to it in a timely fashion certain notes taken by government attorneys during pretrial interviews of Thompson, Moore, and Mrs. Freshour. We disagree.

The Freshour notes were disclosed to Hogan during trial. On appeal, this Court reviewed in camera the Thompson and Moore notes and ordered them disclosed to Hogan in their entirety, with directions that Hogan file a supplemental brief to explain further its position regarding the interview notes. Having considered Hogan's arguments concerning the notes, we hold that Hogan was not prejudiced by their previous unavailability.

A.

The Supreme Court held in Brady v. Maryland, 373 U.S. 83, 83 S.Ct. 1194, 10 L.Ed.2d 215 (1963), that "suppression by the prosecution of evidence favorable to an accused upon request violates due process where the evidence is material either to guilt or to punishment." Id. at 87, 83 S.Ct. at 1196. In Brady, the prosecutor had failed to disclose exculpatory evidence. Hogan makes no plausible claim that the interview notes contain exculpatory material. Rather, Hogan claims that the Thompson and Freshour notes were valuable to its defense for the purpose of impeaching the credibility of Thompson, the government's principle witness. As a recent Supreme Court case explicitly recognizes, however, impeachment evidence can be "favorable to an accused"; "[i]mpeachment evidence, . . . as well as exculpatory evidence, falls within the Brady rule." United States v. Bagley, ___ U.S. ___, ___, 105 S.Ct. 3375, 3380, 87 L.Ed.2d 481 (U.S. July 2, 1985). Accordingly, if either exculpatory or impeachment evidence is suppressed by the prosecution, and if the evidence is material, reversal is warranted.

In cases decided by this Court prior to the Supreme Court's decision in United States v. Bagley,

[w]hether evidence [was] sufficiently material to require its disclosure . . . [was] determined by initially considering whether the defense made a `specific' or merely a `general' request for its disclosure prior to trial. If a specific request was made, evidence [was] deemed material, and a reversal warranted, if disclosure of the evidence `might have affected the outcome of the trial.' If only a general request was made, evidence [was] considered material only if it `created a reasonable doubt that did not otherwise exist [when] evaluated in the context of the entire record.'

Scurr v. Niccum, 620 F.2d 186, 189 (8th Cir. 1980) (quoting United States v. Agurs, 427 U.S. 97, 104, 112, 96 S.Ct. 2392, 2397, 2402, 49 L.Ed.2d 342 (1976)). A majority of the Supreme Court now seems to agree that irrespective of the specificity of a request for evidence made by the defense, for purposes of a Brady inquiry such "evidence is material only if there is a reasonable probability that, had the evidence been disclosed to the defense, the result of the proceeding would have been different." ___ U.S. at ___, 105 S.Ct. at 3383.

We have carefully considered Hogan's claims that it could have used the Thompson and Freshour interview notes to impeach Thompson's credibility at trial and find that Hogan is not entitled to relief under the aegis of Brady. There is not a reasonable probability that the result of Hogan's trial would have been different had the notes been disclosed. The government's failure to produce these notes does not "undermine confidence in the outcome of [Hogan's] trial." Id. at ___, 105 S.Ct. at 3381.

B.

Under the Jencks Act, 18 U.S.C. § 3500, when a witness testifies for the government in a criminal prosecution, the government may, upon request by the defendant and following the witness's direct testimony, be required to produce any statement of the witness which relates to the witness's trial testimony. Hogan made a pretrial request for, and the government voluntarily agreed to produce five days prior to trial, any Jencks Act material in the government's possession. On appeal, Hogan raises a Jencks Act claim. Because this claim seems to be focused upon the Thompson notes, we will discuss it in the context of those notes.

The government apparently concluded that the Thompson notes were not Jencks Act material and did not produce them in advance of trial. When Hogan became aware of the Thompson notes at trial, it moved for their production. The District Court reviewed the Thompson notes in camera, and pursuant to the Jencks Act, ordered extrapolated from the notes and disclosed to Hogan scattered sentences and phrases within the notes that were enclosed in quotation marks. Hogan in fact did not receive this material until after both sides had rested in the presentation of their cases. Hogan now argues that it was prejudiced by the government's failure to make timely disclosure of Jencks Act material.

"Although in many cases the government freely discloses Jencks Act material to the defense in advance of trial, the government may not be required to do so." United States v. White, 750 F.2d 726, 729 (8th Cir. 1984). But when disclosure is ultimately required, it must be made before it is too late for the material to be put to any effective use. We need not address the timeliness of the disclosure in this case because the Thompson notes in their entirety do not as a matter of law constitute Jencks Act material and because Hogan waived any claim it may have had regarding those portions of the Thompson notes ordered disclosed by the District Court.

The Jencks Act, in relevant part, defines a statement as "a stenographic, mechanical, electrical, or other recording, or a transcription thereof, which is a substantially verbatim recital of an oral statement made by [a] witness and recorded contemporaneously with the making of such oral statement." 18 U.S.C. § 3500(e)(2).

Plainly, the Thompson notes when viewed in their entirety do not constitute Jencks Act materials because they are not statements within the meaning of the Jencks Act. See United States v. Cuesta, 597 F.2d 903 (5th Cir.), cert. denied, 444 U.S. 964, 100 S.Ct. 451, 62 L.Ed.2d 377 (1979) (investigators' notes of interviews do not fall under the Jencks Act even if they contain occasional verbatim recitations of phrases of interviewee). As recognized by Justice Powell, "the fact that counsel usually will take notes does not mean that the notes often will be `statements.' Counsel rarely take down verbatim what witnesses say in these preparatory conferences. Consequently, prosecutors' notes may be expected to meet the requirements of [ 18 U.S.C. § 3500(e)(2)] very infrequently." Goldberg v. United States, 425 U.S. 94, 122 n. 9, 96 S.Ct. 1338, 1353 n. 9, 47 L.Ed.2d 603 (1976) (Powell, J., concurring in judgment).

While the District Court did not find that the Thompson notes in their entirety constituted Jencks Act material, it did direct, pursuant to the Jencks Act, that several sentences and phrases enclosed in quotation marks be extracted and given to Hogan. Certainly the District Court was not required to order disclosed as Jencks Act material a few scattered sentences and phrases it found in a body of otherwise non-Jencks Act, attorneys' rough interview notes. In fact, we believe that a trial court ought not employ such a process of extrapolation unless the particular circumstances of a case are extremely compelling. In our view, it is doubtful that the circumstances of this case can fairly be characterized as compelling. We decline, however, to determine whether or not the District Court erred in ordering portions of the Thompson notes disclosed. Cf. Campbell v. United States, 373 U.S. 487, 493, 83 S.Ct. 1356, 1360, 10 L.Ed.2d 501 (1963) (determination by district court of producibility under the Jencks Act may not be disturbed unless clearly erroneous). Regardless of the propriety of the District Court's actions, Hogan waived any Jencks Act claim it may have had regarding the Thompson note extrapolations.

The District Court first ordered the government to disclose portions of the Thompson notes at 1:30 p.m. on May 7, 1984 during an in-chambers conference with counsel. See Tr. at 251. On the morning of May 8, 1984, Hogan rested its case, in spite of the fact and without mentioning to the court that it had not received the material the court ordered disclosed to it on May 7. During the instruction conference, on the afternoon of May 8, 1984, Hogan then complained that it had not received the material. The District Court ruled that Hogan had waived its right to this material by its failure to alert the court to the matter prior to resting its case. The court reasoned that it had no way of knowing that the government had not complied with the court's order and that instead of voluntarily resting its case, Hogan should have requested some time to receive and review the material in order to make a determination as to whether it wished to present additional evidence. See Tr. at 410-11. We hold that the District Court correctly found Hogan's actions to constitute a waiver.

C.

Rule 16(a)(1)(A) of the Federal Rules of Criminal Procedure requires the government, upon request of a defendant, to disclose relevant written or recorded statements of the defendant, as well as the substance of any oral statement made by the defendant in response to interrogation by any person then known by the defendant to be a government agent, which the government intends to offer in evidence at trial. Hogan made a broadly-phrased, pretrial request for production under Rule 16(a)(1)(A). Also prior to trial, Hogan protested to the court that counsel for Moore had informed Hogan that the government possessed notes taken by government attorneys at the time they interviewed Moore and requested that the court order the Moore notes produced. The District Court never explicitly granted or denied this request, but ruled that the discovery proposal of the government, which did not include production of the Moore notes, was acceptable. Hogan now claims that it should have been provided with the Moore notes pursuant to Rule 16(a)(1)(A).

Assuming arguendo that the Moore notes should have been disclosed to Hogan under Rule 16(a)(1)(A), we hold that the non-disclosure of these notes is not a ground for reversal because Hogan's substantial rights were not thereby prejudiced. See United States v. Bledsoe, 674 F.2d 647, 670 (8th Cir.), cert. denied, 459 U.S. 1040, 103 S.Ct. 456, 74 L.Ed.2d 608 (1982). As we already have concluded, it is not reasonably probable that the result of the trial would have been different had the Moore notes been disclosed.

III.

Hogan claims there was a substantial variance between the single conspiracy charged in the indictment, as voluntarily clarified prior to trial by the government, and the proof at trial. We reject this claim.

The indictment charged Hogan with a single conspiracy to rig bids on an unspecified number of highway projects in central Arkansas. Although a formal bill of particulars was never ordered, the government filed a voluntary disclosure two months before trial, specifying three projects on which Hogan was believed to have participated in bid rigging and defined "central Arkansas" as Pulaski County and its adjacent counties. The voluntary disclosure also referred to two to four other unspecified projects on which bids purportedly were rigged with Hogan's cooperation. Hogan contends that it thus was charged with a massive conspiracy to rig five to seven bids on projects in a multi-county area, while the proof adduced at trial related to only three bids on projects in a single county.

Clearly Hogan was, on the basis of the indictment and the government's voluntary disclosure, notified well in advance of trial that it was charged with rigging bids on the three Pulaski County projects to which the proof at trial related, and Hogan had every opportunity to plead to and defend against those charges. The indictment charged Hogan with conspiring to rig bids in violation of the Sherman Act; that charge was not changed by the government's voluntary disclosure and, subject to the interstate commerce problem discussed in Part I of this opinion, was proved at trial. The violation shown was merely narrower than the violation charged. Under these circumstances, Hogan was not prejudiced. See United States v. Miller, ___ U.S. ___, 105 S.Ct. 1811, 1814-16, 85 L.Ed.2d 99 (1985).

IV.

Hogan claims that it was prejudiced by the introduction at trial of A.P.T.'s guilty plea. See supra note 2. Government counsel informed the District Court that it anticipated use by defense counsel of A.P.T.'s guilty plea to impeach Thompson's credibility and proposed that it first be allowed to bring to the jury's attention the matter of A.P.T.'s guilty plea. Tr. at 112, 147-49. Defense counsel objected generally to this course of action, Tr. at 114, but in no way denied that counsel intended to use the guilty plea to attack Thompson's credibility. The District Court ruled that government counsel would be permitted to introduce the fact of A.P.T.'s guilty plea during its direct examination of Thompson.

The following cautionary instruction was given before Thompson testified concerning the guilty plea:

[E]vidence of a witness's own guilty plea, that is, Mr. Thompson for A.P.T. Company, cannot be used except to assess the credibility of the witness and also as an acknowledgment of the witness's participation in the offense charged, but it cannot be considered as substantive evidence of the defendant Ben Hogan Company's guilt, and no inference of guilt as to the Ben Hogan Company can be inferred from A.P.T.'s plea.

Thompson Transcript at 3. Government counsel then questioned Thompson briefly to establish the fact that A.P.T. had pleaded guilty. On cross-examination, in an obvious attempt to impeach Thompson's credibility, counsel for Hogan questioned Thompson extensively about A.P.T.'s guilty plea. Id. at 51-54. In its formal charge to the jury, the District Court again explicitly cautioned the jury that evidence of A.P.T.'s guilty plea could be used only in assessing Thompson's credibility and not in determining Hogan's guilt or innocence. See Tr. at 472-73.

Evidence that [a co-defendant] entered a plea of guilty to the same offense is not error unless it is elicited as substantive proof of the defendant's guilt. Where it is introduced for other purposes, such as to impeach, to reflect on a witness' credibility, or to show the witness' acknowledgment of participation in the offense, evidence of a co-defendant's guilty plea is clearly admissible. In such circumstances, the trial judge should instruct the jury that the evidence is received for this purpose alone and that the plea cannot form the basis of any inference as to the guilt of the defendant.

United States v. Wiesle, 542 F.2d 61, 62 (8th Cir. 1976); cf. Wallace v. Lockhart, 701 F.2d 719, 725-26 (8th Cir.), cert. denied, ___ U.S. ___, 104 S.Ct. 340, 78 L.Ed.2d 308 (1983) (relying on United States v. Wiesle) (co-defendant's prior conviction introduced to show acknowledgment of participation in the crime); United States v. Basic Construction Company, 711 F.2d 570, 574 (4th Cir.), cert. denied, ___ U.S. ___, 104 S.Ct. 371, 527, 78 L.Ed.2d 330 (1983) (given the court's cautionary instruction, no error for court to tell jury of co-defendants' nolo contendere pleas although it is better to simply tell jury that cases of co-defendants had been disposed of without saying how).

We find the introduction of the guilty plea in this case to have been in keeping with United States v. Wiesle and thus within the District Court's discretion. The jury's use of the plea was carefully limited by the District Court to assessment of Thompson's credibility and to A.P.T.'s acknowledgment that Thompson participated in the offense charged. We need not hold the introduction of the plea improper merely because it first was injected into the case by the government on direct examination of Thompson. Cf. United States v. Koppers Company, 652 F.2d 290, 299 (2nd Cir.), cert. denied, 454 U.S. 1083, 102 S.Ct. 639, 70 L.Ed.2d 617 (1981) (references to report which revealed that government's principal witnesses had perjured themselves in early phases of investigation of rigged road construction bids were properly elicited by government on direct examination to anticipate attempts by defense counsel to attack witnesses' credibility). Hogan's claim of prejudice is further eroded by the fact that Hogan's counsel made extensive use of the plea on cross-examination in an attempt to impeach Thompson's testimony. Cf. id.; United States v. Schmaltz, 562 F.2d 558, 560 (8th Cir.), cert. denied, 434 U.S. 957, 98 S.Ct. 485, 54 L.Ed.2d 315 (1977) (where co-defendant pleaded guilty after jury was impaneled and court, without giving a cautionary instruction, informed jury that co-defendant's case had been resolved, defendant suffered no substantial prejudice, especially since defense counsel called co-defendant to the stand and extensively examined him regarding the guilty plea).

V.

Hogan makes additional challenges to the jury instructions given by the District Court, as well as to the District Court's refusal to give two particular instructions proposed by Hogan. We have examined the charge to the jury as a whole and find that Hogan's remaining challenges to the court's instructions are meritless. We also find that the District Court did not abuse its discretion in refusing to instruct the jury as requested by Hogan. Similarly, we reject Hogan's claims of error regarding various other discovery and evidentiary matters. Our review discloses that these matters were entirely within the District Court's soundly exercised discretion.

VI.

Finally, Hogan claims that government counsel improperly injected government counsel's credibility into the trial and misstated the evidence. We have considered these claims and find them to be without merit. However, we caution government counsel on retrial to avoid making statements that could be construed as an injection into the trial of government counsel's credibility or statements of the evidence that are not faithfully accurate. While we do not wish to discourage spirited advocacy, prosecutorial misconduct is a serious matter, and this Court will not allow actual instances of such misconduct to go unremedied.

VII.

With respect to Hogan's conviction on Count One, the judgment of the District Court is reversed and this case is remanded to the District Court for a new trial. With respect to Hogan's convictions on Counts Two through Four, the judgment of the District Court is affirmed.