Anti-chick Thy-1 antibodies administered close to a single trial passive avoidance learning task in day-old chicks resulted in inhibition of long-term memory formation. The retention time course was comparable to that obtained with the protein synthesis inhibitor anisomycin. The amnestic effect of the anti-chick Thy-1 antibody appears to be mediated solely by immunoglobulin G, and is species specific since anti-rat Thy-1 antibodies did not produce amnesia. The results are consistent with a three stage model of memory formation, suggest a role for glycoproteins in long term memory formation, and emphasise the potential of neuroimmunological tools for investigating the neurological bases of memory formation.