Treatment of B6AF1 (C57BL/6 .times. A/Jax) mice with rabbit anti-mouse thymocyte serum (ATS) or with ATS and sheep red blood cells (SRBC) resulted in the generation of splenic suppressor cells within 5 days. Suppressor activity was determined by injecting secondary syngeneic recipients with donor cells and SRBC. The SRBC hemolytic effect of their spleen cells was determined 5 days later. Suppressor cells were found in whole spleen or enriched T [thymus-derived] cell suspensions from mice treated with ATS and SRBC and in only enriched T cell suspensions from mice treated with ATS alone. Apparently ATS induces a population of suppressor cells; these cells may be responsible in part for the immunosuppression noted after treatment with ATS.