Athymic nude mice early in life spontaneously develop a high level of thymocytotoxic antibodies which have characteristics similar to the thymocytotoxic autoantibodies occurring in young NZB mice. Thymocytotoxic activity was no longer detectable in the serum of nude mice two weeks after the intravenous injection of syngeneic thymocytes, although the levels of spontaneously occurring anti-DNA and anti-hapten antibodies were not affected by the cell transfer.