In both studies, undergraduate psychology students played the roles of lawyers and witnesses in a simulation of the pretrial processes of adversary and nonadversary systems. The witnessses (52 males and females in the first study, and 51 males and females in the second study) saw a fight on film, were subsequently interviewed by an adversary or nonadversary lawer (a different lawyer for each subject), and in a trial that took place 1 week after the fight, testified about what they saw. It was predicted that witnesses interviewed by adversary lawyers would bias their testimony in favor of the lawyer's client. The prediction was supported in both studies, although the aggressiveness (Machiavellianism) displayed by the lawyer in the interview proved to be an important variable. Adversary lawyers with low Machiavellian tendencies frequently yielded testimony unfavorable to their clients. The basis transmitted in oral testimony had an influence on the impressions of the factual evidence and responsibility judgments of 'naive' adjudicators, indicating a third-order (lawyer to witness to adjudicator) role-playing effect. The use of untrained 'lawyers' in simulations would suggest that the study results are conservative. Trained lawyers in actual trial situations would tend to be even more persuasive in witness interviews in adversarial legal proceedings. Still, simulations cannot provide conclusive evidence that similar phenomena occur in actual legal procedures, which may contain safeguards against testimonial bias. Results suggest the need for further research. Tabular data, 1 note, and approximately 40 references are provided. (Author abstract modified)