(dissenting). I would award the defendant a new trial because I think it was error, in the circumstances, to exclude evidence that Robinson had reported to the Framingham police, falsely, that he had been threatened and attacked in connection with his plan to testify about another investigation in which he was involved as an undercover agent.
The evidentiary ruling was made at trial in the following circumstances. According to the evidence, the police never observed Robinson while inside the restaurant in which the counterfeit drug transactions allegedly took place, and they never observed the defendant or his vehicle on the restaurant premises around the crucial time. The defendant faced obvious difficulties in mounting any defense other than denial as he was not arrested or alerted to the allegations against him until four months after the alleged incidents. Robinson’s credibility, therefore, was the critical issue. Testimony from Framingham police officers was allowed, over objection, suggesting that Robinson was truthful and well-motivated, and that he had been helpful in other investigations. In this context, Robinson’s testimony about the defendant’s alleged threat was likely to have had a significant impact on the jury. If, on a different occasion before trial, Robinson had fabricated a threat and attack related to his undercover work *44and anticipated testimony for the Framingham police, his testimony that the defendant threatened him in connection with his anticipated testimony in the instant case would be highly suspect.
This is a case in which the right to a full defense was denied because excluded evidence “might have had a significant impact on the result of the trial. . . .” Commonwealth v. Bohannon, 376 Mass. 90, 94 (1978). I do not have to reach the question whether the precise holding in Bohannon is broad enough to encompass the facts of this case, however, because I think the excluded evidence had a rational tendency to prove Robinson’s bias in favor of the police, and reasonable cross-examination for the purpose of showing bias is a matter of right. Compare Commonwealth v. Martinez, 384 Mass. 377, 380 (1981), and cases cited.
The evidence of Robinson’s false claim of threat and injury to the Framingham police suggests a willingness on his part to lie to bolster a police investigation in which he was participating in an undercover capacity. If the jury had known that, for whatever reason, Robinson would go out of his way to fabricate evidence against a suspect, inferably to enhance his standing in the eyes of the Framingham police, they might well, on the basis of that evidence of bias, have had a reasonable doubt whether Robinson had also done so in the instant case.