ERVIN, Circuit Judge:
After exhausting his state remedies, Glenn Edward Hoover, a Maryland prisoner convicted of second degree murder, sought a federal writ of habeas corpus pursuant to 28 U.S.C. § 2254. The district court found that the state trial judge had abridged Hoover’s fourteenth amendment right to confront adverse witnesses by limiting cross-examination of a key prosecution witness. The district court further concluded that [303]*303this error was not harmless, and issued the writ subject to retrial by the state. Maryland appeals, and we now affirm.
I.
Hoover was convicted by a jury in Baltimore County circuit court of second degree murder in connection with an attempted burglary carried out, the state’s evidence showed, by Hoover, Douglas Martin, John Widener, and Paul Greer. Hoover apparently was unarmed during the entire episode, and the murder weapon was fired by Martin. In return for a grant of immunity, Greer turned state’s evidence and was the chief prosecution witness at the separate trials of Widener, who was acquitted, and Hoover, who was convicted. On this appeal, Maryland does not, and cannot, contest seriously Hoover’s contention that Greer’s testimony was vital to the prosecution’s case. The issue before both the district court and this court is whether the state trial court unconstitutionally limited Hoover’s ability to bring out on cross-examination of Greer the concrete details of the agreement Greer made with the government.
In his opening statement the prosecuting attorney informed the jurors that “one of the principal witnesses for the State” was Greer, that Greer had also been charged with the murder, and that he would elaborate later on what Greer was offered in return for his testimony. The prosecutor at that time stated only that “part of the consideration” for Greer’s testimony was a state grant of “immunity from prosecution,” with no explicit specification of what prosecutions the immunity grant covered. In direct examination of Greer later in the trial, the prosecutor asked Greer if he had been charged with the murder, and if he had received “immunity from prosecution ... in this case” in return for testifying, both of which questions Greer answered in the affirmative. The prosecutor then entered into evidence the letter granting Greer immunity, which reads:
Regarding our conversation of January 14, 1974, please be advised that the State will enter a Nol Pros as to all pending charges in Baltimore County against your client, Greer, at the conclusion of the trials of the captioned defendants. It is realized, however, that this action will be taken by the State upon Greer testifying against the three co-defendants in the pending murder case; that is, the testimony that he previously agreed orally to furnish implicating the other three defendants and his entire knowledge of the crime.
The State also agrees to contact the State’s Attorney’s Office in York County, Pennsylvania, in reference to the matter pending against your client in that jurisdiction informing that office as to the results of your client’s testimony in our case.
It is also agreed that no charges will be brought or information released against Greer for any information furnished by him about any other crimes unless they involve physical harm to human beings. [305]*305strate bias, not dishonesty, the traditional rule limiting impeachment to convictions was inapplicable.
[303]*303Greer recounted the events of the crime, portraying himself as an innocent along for a simple stick-up or break-in which turned ugly. Greer testified that when he realized the house to be burglarized was occupied he tried to persuade the others to leave, and, failing in that, insisted that he was not going into the house and would serve only as lookout.
On cross-examination, defense counsel attempted to bring out the details of Greer’s immunity agreement with the state. This attempt was repeatedly frustrated by the trial judge, with the result that only the fact that there was such an agreement, and the bare wording of the immunity letter, were put before the jury. Defense counsel was prevented totally from developing Greer’s understanding of what concrete benefits he would receive from the agreement.1
[305]*305On direct appeal, the Maryland Court of Special Appeals briefly reviewed the issue now before us and concluded that the subject of the immunity agreement was “fully explored” before the jury. Hoover v. State, No. 1065 (Md.Spec.App. June 12, 1975) (unreported). The Maryland Court of Appeals denied Hoover’s petition for review.
II.
The constitutional right of confrontation guaranteed to a state criminal defendant by the fourteenth amendment has as one of its most important aspects the right to cross-examine a hostile witness in order to undermine the credibility of the witness by highlighting the possible influence of bias on the testimony of the witness. See Davis v. Alaska, 415 U.S. 308, 94 S.Ct. 1105, 39 L.Ed.2d 347 (1974). The trial judge may limit such cross-examination only to preserve the witness’ constitutional immunity from self-incrimination, to prevent attempts to harass, humiliate or annoy him, id. at 320, 94 S.Ct. at 1112, or where the information sought might endanger the witness’ personal safety. Chavis v. North Carolina, 637 F.2d 213, 226 (4th Cir.1980). When such factors are not present, substantial limitations on the attempts of a defendant to undermine as biased a witness’ testimony constitute constitutional error.
The trial judge’s traditional discretion to control the limits of cross-examination cannot be exercised until “the constitutionally required threshold level of inquiry has been afforded the defendant.” United States v. Tracey, 675 F.2d 433, 437 (1st Cir.1982). Moreover, any exercise of discretion once that threshold is reached must be informed by “the utmost caution and solicitude for the defendant’s Sixth Amendment rights.” United States v. Houghton, 554 F.2d 1219, 1225 (1st Cir.1977), cert. denied, 434 U.S. 851, 98 S.Ct. 164, 54 L.Ed.2d 120 (1977). “Especially where the witness is an accomplice of the defendant or may have some other substantial reason to cooperate with the government, the defendant should be permitted wide latitude in the search for the witness’ bias.” Tracey, 675 F.2d at 438.
In a decision subsequent to its affirmance of Hoover’s conviction, the Maryland Court of Special Appeals identified what we agree is “the crux of the inquiry” into the credibility of a government witness testifying pursuant to an immunity grant or plea bargain: “the witness’s state of mind. What is essential to the preservation of the right to cross-examine is that the interrogator be permitted to probe into whether the witness is acting under a hope or belief of leniency or reward.” Fletcher v. State, 50 Md.app. 349, 437 A.2d 901, 906 (1981).
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ERVIN, Circuit Judge:
After exhausting his state remedies, Glenn Edward Hoover, a Maryland prisoner convicted of second degree murder, sought a federal writ of habeas corpus pursuant to 28 U.S.C. § 2254. The district court found that the state trial judge had abridged Hoover’s fourteenth amendment right to confront adverse witnesses by limiting cross-examination of a key prosecution witness. The district court further concluded that [303]*303this error was not harmless, and issued the writ subject to retrial by the state. Maryland appeals, and we now affirm.
I.
Hoover was convicted by a jury in Baltimore County circuit court of second degree murder in connection with an attempted burglary carried out, the state’s evidence showed, by Hoover, Douglas Martin, John Widener, and Paul Greer. Hoover apparently was unarmed during the entire episode, and the murder weapon was fired by Martin. In return for a grant of immunity, Greer turned state’s evidence and was the chief prosecution witness at the separate trials of Widener, who was acquitted, and Hoover, who was convicted. On this appeal, Maryland does not, and cannot, contest seriously Hoover’s contention that Greer’s testimony was vital to the prosecution’s case. The issue before both the district court and this court is whether the state trial court unconstitutionally limited Hoover’s ability to bring out on cross-examination of Greer the concrete details of the agreement Greer made with the government.
In his opening statement the prosecuting attorney informed the jurors that “one of the principal witnesses for the State” was Greer, that Greer had also been charged with the murder, and that he would elaborate later on what Greer was offered in return for his testimony. The prosecutor at that time stated only that “part of the consideration” for Greer’s testimony was a state grant of “immunity from prosecution,” with no explicit specification of what prosecutions the immunity grant covered. In direct examination of Greer later in the trial, the prosecutor asked Greer if he had been charged with the murder, and if he had received “immunity from prosecution ... in this case” in return for testifying, both of which questions Greer answered in the affirmative. The prosecutor then entered into evidence the letter granting Greer immunity, which reads:
Regarding our conversation of January 14, 1974, please be advised that the State will enter a Nol Pros as to all pending charges in Baltimore County against your client, Greer, at the conclusion of the trials of the captioned defendants. It is realized, however, that this action will be taken by the State upon Greer testifying against the three co-defendants in the pending murder case; that is, the testimony that he previously agreed orally to furnish implicating the other three defendants and his entire knowledge of the crime.
The State also agrees to contact the State’s Attorney’s Office in York County, Pennsylvania, in reference to the matter pending against your client in that jurisdiction informing that office as to the results of your client’s testimony in our case.
It is also agreed that no charges will be brought or information released against Greer for any information furnished by him about any other crimes unless they involve physical harm to human beings. [305]*305strate bias, not dishonesty, the traditional rule limiting impeachment to convictions was inapplicable.
[303]*303Greer recounted the events of the crime, portraying himself as an innocent along for a simple stick-up or break-in which turned ugly. Greer testified that when he realized the house to be burglarized was occupied he tried to persuade the others to leave, and, failing in that, insisted that he was not going into the house and would serve only as lookout.
On cross-examination, defense counsel attempted to bring out the details of Greer’s immunity agreement with the state. This attempt was repeatedly frustrated by the trial judge, with the result that only the fact that there was such an agreement, and the bare wording of the immunity letter, were put before the jury. Defense counsel was prevented totally from developing Greer’s understanding of what concrete benefits he would receive from the agreement.1
[305]*305On direct appeal, the Maryland Court of Special Appeals briefly reviewed the issue now before us and concluded that the subject of the immunity agreement was “fully explored” before the jury. Hoover v. State, No. 1065 (Md.Spec.App. June 12, 1975) (unreported). The Maryland Court of Appeals denied Hoover’s petition for review.
II.
The constitutional right of confrontation guaranteed to a state criminal defendant by the fourteenth amendment has as one of its most important aspects the right to cross-examine a hostile witness in order to undermine the credibility of the witness by highlighting the possible influence of bias on the testimony of the witness. See Davis v. Alaska, 415 U.S. 308, 94 S.Ct. 1105, 39 L.Ed.2d 347 (1974). The trial judge may limit such cross-examination only to preserve the witness’ constitutional immunity from self-incrimination, to prevent attempts to harass, humiliate or annoy him, id. at 320, 94 S.Ct. at 1112, or where the information sought might endanger the witness’ personal safety. Chavis v. North Carolina, 637 F.2d 213, 226 (4th Cir.1980). When such factors are not present, substantial limitations on the attempts of a defendant to undermine as biased a witness’ testimony constitute constitutional error.
The trial judge’s traditional discretion to control the limits of cross-examination cannot be exercised until “the constitutionally required threshold level of inquiry has been afforded the defendant.” United States v. Tracey, 675 F.2d 433, 437 (1st Cir.1982). Moreover, any exercise of discretion once that threshold is reached must be informed by “the utmost caution and solicitude for the defendant’s Sixth Amendment rights.” United States v. Houghton, 554 F.2d 1219, 1225 (1st Cir.1977), cert. denied, 434 U.S. 851, 98 S.Ct. 164, 54 L.Ed.2d 120 (1977). “Especially where the witness is an accomplice of the defendant or may have some other substantial reason to cooperate with the government, the defendant should be permitted wide latitude in the search for the witness’ bias.” Tracey, 675 F.2d at 438.
In a decision subsequent to its affirmance of Hoover’s conviction, the Maryland Court of Special Appeals identified what we agree is “the crux of the inquiry” into the credibility of a government witness testifying pursuant to an immunity grant or plea bargain: “the witness’s state of mind. What is essential to the preservation of the right to cross-examine is that the interrogator be permitted to probe into whether the witness is acting under a hope or belief of leniency or reward.” Fletcher v. State, 50 Md.app. 349, 437 A.2d 901, 906 (1981). What is crucial is not the government’s understanding of what it is offering in exchange for the testimony. Nor is it what the common law’s “reasonable man” might understand an immunity letter to grant. The vital question, which the defendant is constitutionally entitled to explore by cross-examination, is what the witness understands he or she will receive, for it is this understanding which is of probative value on the issue of bias. The likelihood that a prosecution witness is shading or even contriving testimony adverse to the defendant reasonably can be viewed as directly correlated with the perceived value of such testimony to the witness.
It is this very question, of Greer’s understanding of his stake in testimony adverse to Hoover, which received no answer at Hoover’s trial because of the mistaken2 interventions of the trial judge. Hoover’s counsel was not permitted to inquire into the nature of the Pennsylvania charges in which Maryland had promised to intervene [306]*306in some indeterminate fashion on behalf of Greer. The trial judge took over and effectively curtailed defense counsel’s exploration into other fringe benefits of the immunity arrangement. The judge indicated his belief that the letter granting immunity was the only evidence on the contents of the immunity agreement needed by the jury. The court refused to permit Greer to be questioned about the amount of time in prison he thought he was avoiding by. testifying against Hoover. Greer’s answer to defense counsel’s final question about what other information Greer had supplied the police would have been relevant to indicate the extent of the legal trouble Greer believed himself to be escaping by testifying. The meaning and value of the last paragraph in the immunity letter to Greer3 could not be explored at all because of the trial judge’s interventions.
This sustained and effective refusal to permit inquiry into Greer’s subjective understanding of his bargain with the government stepped beyond the constitutional bounds of the trial court’s discretion, and abridged the fundamental right to confront adverse witnesses secured to Hoover by the fourteenth amendment.
III.
On appeal, the state maintains that even if the trial judge’s actions were constitutional error, the error must be judged harmless in the context of this case.4
“Harm is presumed to have come from the constitutional error, and the state has the ‘heavy burden’ of proving harmlessness beyond a reasonable doubt.” Anderson v. Warden, 696 F.2d 296, 300 (4th Cir.1982) (en banc) (citation omitted). The state has not carried its “heavy burden” in the present case. The state trial court’s interference with Hoover’s counsel’s attempts to undermine Greer’s credibility was pervasive and the issues which defense counsel sought to develop were highly relevant. Greer was the key to the prosecution case, and if the jury doubted his story there would have been little else on which to base a guilty verdict. None of the factors justifying the limitation of cross-examination in this type of situation (a need to safeguard the witness’ constitutional immunity from self-incrimination, or to protect the witness from attempts to harass or humiliate or from the possibility of physical harm) were present.
Finally, it is significant that Widener, the leader of the criminal enterprise according to the state’s version of the facts, was acquitted after a trial in which defense counsel was permitted to question Greer about the details of his immunity agreement, while Hoover, in whose trial those details remained unexplored, was convicted.
[307]*307Given these facts, we cannot conclude beyond a reasonable doubt “that there was no reasonable possibility that the error might have contributed to the conviction.” Cox v. State, 51 Md.App. 271, 443 A.2d 607, 614 (1982) (emphasis in original).
IV.
We agree with the district court’s conclusions that Hoover’s trial was marred by substantial constitutional error, and that the state has not shown this error to be harmless beyond a reasonable doubt. The judgment of the district court is affirmed.
AFFIRMED.