In the transcript the PhD testified for the defense and the MD for the prosecution or the MD testified for the defense and the PhD for the prosecution. Subjects rated both experts on 10 items related to witness credibility and gave verdicts of guilty, not guilty by reason of insanity (NGRI), or guilty. Results indicated a medical bias in that subjects tended to follow the MD's recommendations, endorse attitudes consistent with the MD's testimony, and rate the MD as more credible; but only in the PhD-defense/MD-prosecution condition. Specifically, anti-insanity defense subjects in this condition found the defendant guilty of both charges, consistent with the MD position. In contrast, when the MD testified for the defense, there were approximately equal numbers of NGRI and guilty verdicts. For the burglary charge in the PhD-defense/MD-prosecution condition, low issue-involved subjects most consistently concurred with the position of the MD. A medical bias also was found on the credibility measures, with the MD consistently viewed as more credible in the MD-prosecution condition. In the MD-defense condition, subjects found the witnesses equally trustworthy and credible. 3 tables and 38 references. (Author abstract modified)