delivered the opinion of the Court.

The Due Process Clause of the Fourteenth Amendment requires the State to disclose to criminal defendants favorable evidence that is material either to guilt or to punishment. United States v. Agurs, 427 U. S. 97 (1976); Brady v. *481Maryland, 373 U. S. 83 (1963). This case raises the question whether the Fourteenth Amendment also demands that the State preserve potentially exculpatory evidence on behalf of defendants. In particular, the question presented is whether the Due Process Clause requires law enforcement agencies to preserve breath samples of suspected drunken drivers in order for the results of breath-analysis tests to be admissible in criminal prosecutions.

f — I

The Omicron Intoxilyzer (Intoxilyzer) is a device used in California to measure the concentration of alcohol in the blood of motorists suspected of driving while under the influence of intoxicating liquor.1 The Intoxilyzer analyzes the suspect’s breath. To operate the device, law enforcement officers follow these procedures:

“Prior to any test, the device is purged by pumping clean air through it until readings of 0.00 are obtained. The breath test requires a sample of‘alveolar’ (deep lung) air; to assure that such a sample is obtained, the subject is required to blow air into the intoxilyzer at a constant pressure for a period of several seconds. A breath sample is captured in the intoxilyzer’s chamber and infrared light is used to sense the alcohol level. Two samples are taken, and the result of each is indicated on a printout card. The two tests must register within 0.02 of each other in order to be admissible in court. After each test, the chamber is purged with clean air and then *482checked for a reading of zero alcohol. The machine is calibrated weekly, and the calibration results, as well as a portion of the calibration samples, are available to the defendant.” 142 Cal. App. 3d 138, 141-142, 190 Cal. Rptr. 319, 321 (1983) (citations omitted).

In unrelated incidents in 1980 and 1981, each of the respondents in this case was stopped on suspicion of drunken driving on California highways. Each respondent submitted to an Intoxilyzer test.2 Each respondent registered a blood-alcohol concentration substantially higher than 0.10 percent. Under California law at that time, drivers with higher than 0.10 percent blood-alcohol concentrations were presumed to be intoxicated. Cal. Veh. Code Ann. § 23126(a)(3) (West 1971) (amended 1981). Respondents were all charged with driving while intoxicated in violation of Cal. Veh. Code Ann. §23102 (West 1971) (amended 1981).

Prior to trial in Municipal Court, each respondent filed a motion to suppress the Intoxilyzer test results on the ground that the arresting officers had failed to preserve samples of respondents' breath. Although preservation of breath samples is technically feasible,3 California law enforcement offi*483cers do not ordinarily preserve breath samples, and made no effort to do so in these cases. Respondents each claimed that, had a breath sample been preserved, he would have been able to impeach the incriminating Intoxilyzer results. All of respondents’ motions to suppress were denied. Respondents Ward and Berry then submitted their cases on the police records and were convicted. Ward and Berry subsequently petitioned the California Court of Appeal for writs of habeas corpus. Respondents Trombetta and Cox did not submit to trial. They sought direct appeal from the Municipal Court orders, and their appeals were eventually transferred to the Court of Appeal to be consolidated with the Ward and Berry petitions.4

The California Court of Appeal ruled in favor of respondents. After implicitly accepting that breath samples would be useful to respondents’ defenses, the court reviewed the available technologies and determined that the arresting officers had the capacity to preserve breath samples for respondents. 142 Cal. App. 3d, at 141-142, 190 Cal. Rptr., at 320-321. Relying heavily on the California Supreme Court’s decision in People v. Hitch, 12 Cal. 3d 641, 527 P. 2d 361 (1974), the Court of Appeal concluded: “Due process demands simply that where evidence is collected by the state, as it is with the intoxilyzer, or any other breath testing device, law enforcement agencies must establish and follow rigorous and *484systematic procedures to preserve the captured evidence or its equivalent for the use of the defendant.” 142 Cal. App. 3d, at 144, 190 Cal. Rptr., at 323.5 The court granted respondents Ward and Berry new trials, and ordered that the Intoxilyzer results not be admitted as evidence against the other two respondents. The State unsuccessfully petitioned for certiorari in the California Supreme Court, and then petitioned for review in this Court. We granted certiorari, 464 U. S. 1037 (1984), and now reverse.

*485II

Under the Due Process Clause of the Fourteenth Amendment, criminal prosecutions must comport with prevailing notions of fundamental fairness. We have long interpreted this standard of fairness to require that criminal defendants be afforded a meaningful opportunity to present a complete defense. To safeguard that right, the Court has developed “what might loosely be called the area of constitutionally guaranteed access to evidence.” United States v. Valenzuela-Bernal, 458 U. S. 858, 867 (1982). Taken together, this group of constitutional privileges delivers exculpatory evidence into the hands of the accused, thereby protecting the innocent from erroneous conviction and ensuring the integrity of our criminal justice system.

The most rudimentary of the access-to-evidence cases impose upon the prosecution a constitutional obligation to report to the defendant and to the trial court whenever government witnesses lie under oath. Napue v. Illinois, 360 U. S. 264, 269-272 (1959); see also Mooney v. Holohan, 294 U. S. 103 (1935). But criminal defendants are entitled to much more than protection against perjury. A defendant has a constitutionally protected privilege to request and obtain from the prosecution evidence that is either material to the guilt of the defendant or relevant to the punishment to be imposed. Brady v. Maryland, 373 U. S., at 87. Even in the absence of a specific request, the prosecution has a constitutional duty to turn over exculpatory evidence that would raise a reasonable doubt about the defendant’s guilt. United States v. Agurs, 427 U. S., at 112. The prosecution must also reveal the contents of plea agreements with key government witnesses, see Giglio v. United States, 405 U. S. 150 (1972), and under some circumstances may be required to disclose the identity of undercover informants who possess evidence critical to the defense, Roviaro v. United States, 353 U. S. 53 (1957).

*486Less clear from our access-to-evidence cases is the extent to which the Due Process Clause imposes on the government the additional responsibility of guaranteeing criminal defendants access to exculpatory evidence beyond the government’s possession. On a few occasions, we have suggested that the Federal Government might transgress constitutional limitations if it exercised its sovereign powers so as to hamper a criminal defendant’s preparation for trial. For instance, in United States v. Marion, 404 U. S. 307, 324 (1971), and in United States v. Lovasco, 431 U. S. 783, 795, n. 17 (1977), we intimated that a due process violation might occur if the Government delayed an indictment for so long that the defendant’s ability to mount an effective defense was impaired. Similarly, in United States v. Valenzuela-Bernal, supra, we acknowledged that the Government could offend the Due Process Clause of the Fifth Amendment if, by deporting potential witnesses, it diminished a defendant’s opportunity to put on an effective defense.6 458 U. S., at 873.

We have, however, never squarely addressed the government’s duty to take affirmative steps to preserve evidence on behalf of criminal defendants. The absence of doctrinal development in this area reflects, in part, the difficulty of developing rules to deal with evidence destroyed through prosecutorial neglect or oversight. Whenever potentially exculpatory evidence is permanently lost, courts face the treacherous task of divining the import of materials whose contents are unknown and, very often, disputed. Cf. United States v. Valenzuela-Bernal, supra, at 870. Moreover, fashioning remedies for the illegal destruction of evidence can pose troubling choices. In nondisclosure cases, a court can *487grant the defendant a new trial at which the previously suppressed evidence may be introduced. But when evidence has been destroyed in violation of the Constitution, the court must choose between barring further prosecution or suppressing — as the California Court of Appeal did in this case— the State’s most probative evidence. '

One case in which we have discussed due process constraints on the Government’s failure to preserve potentially exculpatory evidence is Killian v. United States, 368 U. S. 231 (1961). In Killian, the petitioner had been convicted of giving false testimony in violation of 18 U. S. C. § 1001. A key element of the Government’s case was an investigatory report prepared by the Federal Bureau of Investigation. The Solicitor General conceded that, prior to petitioner’s trial, the F. B. I. agents who prepared the investigatory report destroyed the preliminary, notes they had made while interviewing witnesses. The petitioner argued that these notes would have been helpful to his defense and that the agents had violated the Due Process Clause by destroying this exculpatory evidence. While not denying that the notes might have contributed to the petitioner’s defense, the Court ruled that their destruction did not rise to the level of constitutional violation:

“If the agents’ notes . . . were made only for the purpose of transferring the data thereon . . . , and if, having served that purpose, they were destroyed by the agents in good faith and in accord with their normal practices, it would be clear that their destruction did not constitute an impermissible destruction of evidence nor deprive petitioner of any right.” Id., at 242.

In many respects the instant case is reminiscent of Killian v. United States. To the extent that respondents’ breath samples came into the possession of California authorities, it was for the limited purpose of providing raw data to the *488Intoxilyzer.7 The evidence to be presented at trial was not the breath itself but rather the Intoxilyzer results obtained from the breath samples. As the petitioner in Killian wanted the agents’ notes hi order to impeach their final reports, respondents here seek the breath samples in order to challenge incriminating tests results produced with the Intoxilyzer.

Given our precedents in this area, we cannot agree with the California Court of Appeal that the State’s failure to retain breath samples for respondents constitutes a violation of the Federal Constitution. To begin with, California authorities in this case did not destroy respondents’ breath samples in a calculated effort to circumvent the disclosure requirements established by Brady v. Maryland and its progeny. In failing to preserve breath samples for respondents, the officers here were acting “in good faith and in accord with their normal practice.” Killian v. United States, supra, at 242. The record contains no allegation of official animus towards respondents or of a conscious effort to suppress exculpatory evidence.

More importantly, California’s policy of not preserving breath samples is without constitutional defect. Whatever duty the Constitution imposes on the States to preserve evidence, that duty must be limited to evidence that might be expected to play a significant role in the suspect’s defense.8 *489To meet this standard of constitutional materiality, see United States v. Agurs, 427 U. S., at 109-110, evidence must both possess an exculpatory value that was apparent before the evidence was destroyed, and be of such a nature that the defendant would be unable to obtain comparable evidence by other reasonably available means. Neither of these conditions is met on the facts of this case.

Although the preservation of breath samples might conceivably have contributed to respondents’ defenses, a dispassionate review of the Intoxilyzer and the California testing procedures can only lead one to conclude that the chances are extremely low that preserved samples would have been exculpatory. The accuracy of the Intoxilyzer has been reviewed and certified by the California Department of Health.9 To protect suspects against machine malfunctions, the Department has developed test procedures that include two independent measurements (which must be closely correlated for the results to be admissible) bracketed by blank runs designed to ensure that the machine is purged of alcohol traces from previous tests. See supra, at 481-482. In all but a tiny fraction of cases, preserved breath samples would simply confirm the Intoxilyzer’s determination that the defendant had a high level of blood-alcohol concentration at the time of the test. Once the Intoxilyzer indicated that respondents were legally drunk, breath samples were much more likely to provide inculpatory than exculpatory evidence.10

*490Even if one were to assume that the Intoxilyzer results in this case were inaccurate and that breath samples might therefore have been exculpatory, it does not follow that respondents were without alternative means of demonstrating their innocence. Respondents and amici have identified only a limited number of ways in which an Intoxilyzer might malfunction: faulty calibration, extraneous interference with machine measurements, and operator error. See Brief for Respondents 32-34; Brief for California Public Defender’s Association et al. as Amici Curiae 25-40. Respondents were perfectly capable of raising these issues without resort to preserved breath samples. To protect against faulty calibration, California gives drunken driving defendants the opportunity to inspect the machine used to test their breath as well as that machine’s weekly calibration results and the breath samples used in the calibrations. See supra, at 481-482. Respondents could have utilized these data to impeach the machine’s reliability. As to improper measurements, the parties have identified only two sources capable of interfering with test results: radio waves and chemicals that appear in the blood of those who are dieting. For defendants whose test results might have been affected by either of these factors, it remains possible to introduce at trial evidence demonstrating that the defendant was dieting at the time of the test or that the test was conducted near a source of radio waves. Finally, as to operator error, the defendant retains the right to cross-examine the law enforcement officer who administered the Intoxilyzer test, and to attempt to raise doubts in the mind of the factfinder whether the test was properly administered.11

*491H-1 I — H

We conclude, therefore, that the Due Process Clause of the Fourteenth Amendment does not require that law enforcement agencies preserve breath samples in order to introduce the results of breath-analysis tests at trial.12 Accordingly, the judgment of the California Court of Appeal is reversed, and the case is remanded for further proceedings not inconsistent with this opinion.

It is so ordered.

concurring.

Rules concerning preservation of evidence are generally matters of state, not federal constitutional, law. See United States v. Augenblick, 393 U. S. 348, 352-353 (1969). The failure to preserve breath samples does not render a prosecution fundamentally unfair, and thus cannot render breath-analysis tests inadmissible as evidence against the accused. Id., at 356. Similarly, the failure to employ alternative methods of testing blood-alcohol concentrations is of no due *492process concern, both because persons are presumed to know their rights under the law and because the existence of tests not used in no way affects the fundamental fairness of the convictions actually obtained. I understand the Court to state no more than these well-settled propositions. Accordingly, I join both its opinion and judgment.