¶1OPINION
¶2Robert E. Jackson and Jimmie Lee Eacker were jointly indicted for first-degree robbery, AS 11.41.500(a)(1). Eacker pled no contest and testified against Jackson at Jackson’s trial prior to being sentenced. The jury convicted Jackson. Jackson appeals. We reverse.
¶3FACTS
¶4On January 28, 1982, at approximately 1:44 a.m., the Time Saver Grocery in Kenai was robbed at gunpoint. Linda Bartels was working there at the time. Officer Michael Anastay arrived within five minutes with his specially trained police dog. The dog followed the robber’s scent to the rear of the store, where Officer Anastay found fresh tire tracks and footprints in the snow. The footprints ended alongside the right side of the tire tracks, where the passenger side would have been. The tracks were measured and photographed.
¶5Sergeant Dorcas followed the tracks to the Rock and Roll Clinic, a night club, where a vehicle was found with tires that matched the tracks. The vehicle belonged to Jeff Johnson. Johnson, Jackson and Eacker were all at the club when the officers arrived. On April 9, 1982, Eacker contacted the police, confessed to the robbery, and implicated Jackson as the driver of the getaway car.
¶6At trial, Jeff Johnson testified that he had loaned Jackson his car at about 6:00 p.m. on the night of the robbery. Jackson and Eacker had helped a friend, Sherry Betz, and her roommate move some furniture between 6:00 p.m. and 9:00 p.m. Both Betz and Jerry Carlson, a bouncer at the *229Rock and Roll Clinic, testified that they thought Jackson had arrived at the club around 11:00 p.m. Later in the evening Betz noticed that Jackson and Eacker had left the Rock and Roll Clinic, but they returned some time before closing time. Betz also said that Jackson had tried to persuade her to provide him with an alibi by saying that she had been with him all night, but she refused. Jackson testified that sometime between 11:00 p.m. and midnight he had been parked next to Doyle’s Fuel Service warehouse just past the Time Saver Grocery on the North Road, waiting to see if Eadie of Eadie’s Pawn Shop might show up so that Jackson could pawn a CB radio. He waited about ten or fifteen minutes, then went to the Rock and Roll Clinic. He denied participation in the Time Saver robbery.
¶7Prior to trial, Jackson learned that his codefendant, Eacker, was the prime suspect in a murder case that was then under investigation. Immediately before confessing his involvement in the Time Saver robbery, Eacker had been questioned by the police about the disappearance of Toni Lister. Lister’s body was found in Seward on April 17, 1982, a week after Eacker’s confession to the robbery. Jackson filed a pretrial discovery motion requesting police reports of the pending murder investigation. Though the court denied discovery to Jackson, the state agreed to permit defense counsel to examine the reports for a few hours in an in camera setting. It was Jackson’s intent to use the reports to discredit Eacker’s testimony by showing his bias or motive to cooperate with the police on the robbery in hopes of obtaining leniency on the murder charge.
¶8During voir dire Jackson was not allowed to question jurors about their knowledge of the Lister homicide. However, two jurors volunteered their knowledge of Eacker’s possible connection to the Seward murder. The state challenged these jurors for cause, and they were excused.
¶9At trial, over defense objection, the court issued a protective order precluding counsel from mentioning that Eacker was the suspect in a Seward homicide investigation. Judge Cranston ruled that Jackson would be permitted to cross-examine Eacker only as to whether he had been questioned by police about another “major felony matter” at the time of his confession.
¶10However, Eacker repeatedly invoked the fifth amendment when asked about his involvement in the other “major felony.” Eacker declined to acknowledge that he had become involved in a police investigation of a major felony. After Jackson challenged Eacker’s repeated invocation of the fifth amendment, Judge Cranston conducted an in camera hearing with Eacker’s counsel. Following the hearing, Judge Cranston ruled that Eacker’s invocation of the fifth amendment with respect to questions concerning the Lister murder investigation was valid.
¶11Judge Cranston did allow Jackson’s counsel to continue questioning Eacker about his involvement as a suspect in another “major felony matter,” but Eacker was permitted to invoke the fifth amendment in response to all such questions. Eacker declined to answer whether he knew he was under investigation for another felony at the time he confessed to the Time Saver robbery or whether he believed that he could help himself in another case by cooperating on the Time Saver incident. Eacker acknowledged that, before his confession, he specifically requested to speak with Lieutenant Mahurin of the Kenai Police Department because he had dealt with Mahurin in the past, had a friendly relationship with him, and believed Mahurin could help with his problem. However, Eacker invoked the fifth amendment when asked if the problem they had talked about included another felony. Eacker further declined to answer when asked if he had made any statement to Mahurin about another major felony matter before confessing to the robbery.
¶12Lieutenant Mahurin testified that on the afternoon of April 9 he had been informed that Eacker was at the Kenai Police Department and had asked to speak with him. Mahurin went to the station and first *230talked with Eacker “regarding another pending investigation.” Mahurin testified that he then “told Mr. Eacker that I was glad that he had come in, that I had been wanting to speak to him about something anyway, and told him that what I wanted to speak to him about was the robbery at the Time Saver Grocery Store.” Under cross-examination Mahurin stated that, pri- or to taking Eacker’s confession for the robbery, he had discussed the “other felony” and other matters with Eacker for approximately twenty-five minutes. Mahu-rin was not questioned further about the other felony matter.
¶13DISCUSSION
¶14Jackson’s primary argument is that he was denied his constitutional right to confront and cross-examine Eacker by putting Eacker’s confession in its proper setting and demonstrating to the jury Eacker’s possible bias: his hope that by confessing to the Time Saver robbery and implicating Jackson, he might influence the outcome of the murder investigation in the Lister case. Jackson relies primarily on Davis v. Alaska, 415 U.S. 308, 94 S.Ct. 1105, 39 L.Ed.2d 347 (1974).
A more particular attack on the witness’ credibility is effected by means of cross-examination directed toward revealing possible biases, prejudices, or ulterior motives of the witness as they may relate directly to issues or personalities in the case at hand. The partiality of a witness is subject to exploration at trial, and is “always relevant as discrediting the witness and affecting the weight of his testimony.” 3A J. Wigmore Evidence § 940, p. 775 (Chadbourn rev. 1970). We have recognized that the exposure of a witness’ motivation in testifying is a proper and important function of the constitutionally protected right of cross-examination.
¶15Thus, cross-examination as to bias is not deemed collateral, and refusal of a prosecution witness, on fifth amendment grounds, to answer questions relating to bias will implicate the defendant’s confrontation right. See, e.g., United States v. Gambler, 662 F.2d 834 (D.C.Cir.1981). Under such circumstances, the trial court is required to strike relevant testimony given by the witness on direct examination unless the specific subject matter addressed by the question as to which the privilege has been invoked is cumulative or remote, or unless the defendant is afforded an adequate independent means to establish the witness’ bias. See, e.g., United States v. Brown, 634 F.2d 819 (5th Cir.1981); United States v. Seifert, 648 F.2d 557 (9th Cir. *2311980); Fountain v. United States, 384 F.2d 624 (5th Cir.1967), cert. denied, 390 U.S. 1005, 88 S.Ct. 1246, 20 L.Ed.2d 105 (1968).
¶16The vital question in each case is whether the defendant’s fundamental right to challenge the accuracy of a witness’ testimony through cross-examination has been substantially impaired.
The ultimate inquiry is whether the defendant has been deprived of his right to test the truth of the direct testimony. If he has, so much of the direct testimony as cannot be subjected to sufficient inquiry must be struck. The distinction is generally drawn between invoking the privilege as to “collateral matters,” not requiring the striking of direct testimony, and invoking it as to “direct” matters. But the line between “direct” and “collateral” is not clear, and the question in each case must finally be whether defendant’s inability to make the inquiry created a substantial danger of prejudice by depriving him of the ability to test the truth of the witness’s direct testimony.
¶17Fountain v. United States, 384 F.2d 624, 628 (5th Cir.1967) (citations omitted), cert. denied, 390 U.S. 1005, 88 S.Ct. 1246, 20 L.Ed.2d 105 (1968). See also United States v. Brown, 634 F.2d at 824-25; United States v. Diecidue, 603 F.2d 535, 551-52 (5th Cir.1979), cert. denied, 445 U.S. 946, 100 S.Ct. 1345, 63 L.Ed.2d 781 (1980).
¶18In the present case, we conclude that Eacker’s invocation of his fifth amendment right as to matters relating to the Lister murder investigation deprived Jackson of his right to confrontation. Eacker invoked the fifth amendment as to virtually all questions concerning the possible impact of the Lister murder investigation on his motivation for confessing the Time Saver robbery and for implicating Jackson. These questions bore directly on the issue of Eacker’s motivation for implicating Jackson, i.e., his bias, and were not collateral.
¶19Nor can the questions fairly be deemed cumulative of testimony tending to show that Eacker might have been motivated by a desire to obtain favorable treatment for himself for his role in the Time Saver robbery. The relevance of the Lister murder evidence was not to demonstrate that Eacker might be subject to the level of bias reasonably attributable to an accomplice. Rather, its relevance was to show an independent and potentially far more influential source of bias. In this regard, it is simply beside the point that Eacker’s status as an accomplice was demonstrated and that the jury was instructed accordingly.
¶20Evidence that Eacker was a prime suspect in the Lister murder and that the murder investigation was pending at the time of his confession on April 9 would have effectively added a new dimension to the issue of bias. Unless the jury was aware that Eacker knew he was a suspect in the Lister murder investigation, it could easily have concluded that Eacker had no reason to confess his own complicity in the Time Saver robbery other than a desire to be truthful and forthright.
¶21Under these circumstances, full cross-examination as to Eacker’s knowledge of the Lister murder investigation was essential, since the questioning could have demonstrated that when Eacker confessed to the Time Saver robbery he had substantially more to gain than the possibility of favorable treatment on the robbery charge. Had full cross-examination been allowed, Jackson could have directly called into *232question Eacker’s motivation for confessing to participation in the robbery. The jury would have been presented with a substantial reason to question whether Eaeker could be viewed as a basically honest witness. Thus, the jury might have been far more receptive to Jackson’s argument that Eacker’s confession and his sub-, sequent testimony were false.
¶22Under the circumstances, we think the holding of the court in Davis v. Alaska is apt:
We cannot speculate as to whether the jury, as sole judge of the credibility of a witness, would have accepted this line of reasoning had counsel been permitted to fully present it. But we do conclude that the jurors were entitled to have the benefit of the defense theory before them so that they could make an informed judgment as to the weight to place on Green’s [the witness’] testimony which provided “a crucial link in the proof ... of petitioner’s act.”
¶23Davis, 415 U.S. at 317, 94 S.Ct. at 1111, 39 L.Ed.2d at 354 (citation omitted).
¶24We conclude that, in the circumstances of this case, evidence of Eacker’s possible bias stemming from the Lister murder investigation cannot fairly be deemed cumulative of other evidence indicating possible bias.
¶25Upon review of the entire record, we are satisfied that Eacker’s invocation of the fifth amendment privilege as to virtually all questions dealing with the Lister murder investigation “created a substantial danger of prejudice by depriving [Jackson] of the ability to test the truth of [Eacker’s] direct testimony.” Fountain v. United States, 384 F.2d at 628. Accordingly, we hold that Jackson’s inability to question Eaeker fully as to bias constituted a deprivation of Jackson’s right to confrontation, requiring reversal of the conviction.
¶26*233We must separately consider Jackson’s claim that Eacker’s testimony was not sufficiently corroborated to satisfy AS 12.45.020.
¶27The judgment of the superior court is REVERSED and this case is REMANDED for a new trial.
¶28SINGLETON, J., dissents.
¶29. He also relies on a number of Alaska cases that deal with the scope of the constitutional right to confrontation. See Coffey v. State, 585 P.2d 514, 522-23 (Alaska 1978); Evans v. State, 550 P.2d 830, 834-41 (Alaska 1976); Hutchings v. State, 518 P.2d 767 (Alaska 1974); Fields v. State, 487 P.2d 831, 845-46 (Alaska 1971); R.L.R. v. State, 487 P.2d 27, 43-44 (Alaska 1971); and Whitton v. State, 479 P.2d 302, 316-18 (Alaska 1970). These cases stand for the general proposition that the trial court violates a defendant’s constitutional right of confrontation if it unduly restricts his cross-examination of prosecution witnesses concerning possible motives for falsely accusing the defendant.
¶30. On appeal the parties have provided no information concerning the current status of the Lister murder investigation or of Eacker’s actual involvement in that offense. Accordingly, discussion of the circumstances disclosed by Eacker’s counsel during the in camera hearing before Judge Cranston would be inappropriate in this opinion. It is sufficient to note, however, that nothing stated in the in camera proceedings would support a conclusion that the pendency of the murder investigation did not play a role in Eacker's decision to confess to the Time Saver robbery.
¶31. The holding of Robinson v. State, 593 P.2d 621, 623-24 (Alaska 1979), is inapposite to the present case. In Robinsonthe defendant was deprived of the opportunity to cross-examine the state’s witness as to whether he had actually committed a crime for which charges were then pending against him in the state of California. Although the witness invoked the fifth amendment, the defendant was able to establish the existence and nature of the pending charges, and the witness admitted that he knew of the fact that charges were pending. Moreover, the witness admitted other criminal conduct from which a desire to curry favor with the police could have been inferred. Under those circumstances, it is apparent that cross-examination of the witness concerning actual commission of the California offense would have added nothing significant to the evidence of bias that already existed in the record.
¶32. In reaching our conclusion, we reject the notion that Jackson’s ability to argue bias from Eacker’s repeated invocation of the fifth amendment constituted an adequate substitute for cross-examination. It would have been impermissible for the jury to draw inferences from Eacker’s invocation of the fifth amendment, and it would be improper for this court to presume that the jury drew such inferences. Jackson’s counsel, in arguing the case to the jury, could not expressly rely on Eacker’s invocation of the fifth amendment. Similarly, since questions asked by counsel during the trial did not constitute evidence, the jury could not properly have relied on facts inferrable from Jackson’s cross-examination as a source for inferring Eacker’s bias.
¶33In any event, it is apparent that the possibility that the jury might have drawn inferences concerning Eacker’s state of mind from his invocation of the fifth amendment would not constitute an adequate substitute for cross-examination:
Confrontation means more than being allowed to confront the witness physically. "Our cases construing the [confrontation] clause hold that a primary interest secured by it is the right of cross-examination."
¶34Davis v. Alaska, 415 U.S. at 315, 94 S.Ct. at 1110, 39 L.Ed.2d at 353, quoting Douglas v. Alabama, *233380 U.S. 415, 418, 85 S.Ct. 1074, 1076, 13 L.Ed.2d 934 (1965).
¶35. Our disposition of the confrontation issue makes it unnecessary for us to decide the other issues raised on appeal by Jackson, which concern his inability to obtain discovery of materials dealing with the Lister murder investigation. In the event controversy should again arise on remand, the court should reevaluate the question in light of the current status of the Lister murder investigation and the present need for confidentiality.