Rabbit seminal plasma contains at least one isoantigenically active component. No isoantigenicity could be observed with rabbit spermatozoa, even though spermatozoa remove the homologous antibody against seminal plasma. Rabbit spermatozoa or seminal plasma injected in Freund''s adjuvant into rabbits do not evoke degenerative changes in rabbit testis or epididymis. Normal rabbit serum fix complement with rabbit spermatozoa.