The natural autoantibody against thymocytes of NZB mice was found to be cytotoxic in the presence of complement for some lymphocytes in the peripheral lymphoid tissues and in the circulation of C57BL/6J mice and phenotypically normal mice heterozygous for the recessive nude gene (nu/+). An extreme reduction or a total depletion in the number of autoantibody-sensitive lymphocytes was observed both in C57BL/6J mice thymectomized as newborns or as adults and in congenitally athymic nude mice homozygous for the nude gene (nu/nu). This observation suggests that the natural autoantibody against thymocytes of NZB mice has specificity for thymus-dependent cells. Congenitally athymic mice, although lacking autoantibody-sensitive lymphocytes, showed reactive antigen in brain tissue.