Naturally occurring thymocytotoxic autoantibody (NTA) was detected by cytotoxic test in the sera of very young New Zealand Black mice (within 1 month after birth); the incidence was 100% at 3 months of age. Some mice from other strains also had NTA, but at an older age and with lower incidence and antibody titer. NTA had optimal activity at 4 degrees C but was also strongly reactive at 37 degrees C. It was cytotoxic for thymocytes of all strains of mice tested. Whereas only thymocytes were highly sensitive to NTA, the reactive antigen was demonstrated by absorption test in the thymus, lymph node, spleen, and brain of adult mice. It could be demonstrated only in the thymus of newborn mice.The distribution of NTA-reactive antigen suggests the presence of an antigen distinct from any so far described on the cell surface of mouse thymocytes. Gel filtration of NZB mouse serum suggests that NTA is an IgM. Mouse thymocytes sensitized with NTA in vitro became highly susceptible to phagocytosis by syngeneic macrophages.