One-day-old chicks trained on a single trial passive avoidance task were administered a monoclonal anti-chick Thy-1 antibody, either intracranially or subcutaneously, at various times before and after learning and retention tested at various times post-learning. This procedure resulted in profound amnesia when anti-Thy-1 antibody was administered immediately before learning (5 min) in the case of the subcutaneous injections or 5 min before until 5 min after the learning process with intracranial injections. Antibody administered at other times, either before or after learning had little or no effect on retention. Retention levels were normal until 50 min post-learning, then declined sharply and remained at control levels for the duration of the test period. Chicks injected with anti-chick cerebellum or anti-rat Thy-1 antibodies showed no evidence of amnesia for the concentration of the antibodies used.