Thymocytes or spleen cells were transferred from normal adult Lewis rats that had received 100 mg bovine γ-globulin intraperitoneally to intact adult syngeneic recipients. The recipients, after challenge at 24 hr with antigen in complete adjuvant, showed inhibition of the Arthus and delayed hypersensitivity responses and a slight delay in the formation of hemagglutinating and hemolytic antibody but not anaphylactic antibody. The effect was specific and apparently was not due to transferred antigen (transfer of killed thymocytes or spleen cells had no effect); it was observed between 18 and 72 hr after the injection of bovine γ-globulin in donor animals.