Summary. The effect on spermatogenesis of intratesticular and subcutaneous immunization with genital tract fluids of bulls was studied in sexually mature guinea-pigs, rabbits and sheep. Immunization with bovine seminal vesicle fluid produced a decrease in the index weight of the testes, a disturbance of spermatogenesis and, in more severe cases, a distortion and contraction of the seminiferous tubules. In all animals immunized subcutaneously, identical circulating antibodies were found in animals with damaged testes and in animals with normal testes. They did not cause spermagglutination, immobiliza- tion or spermatotoxic reactions. No circulating antibodies could be found in guinea-pigs and rabbits immunized by a single intratesticular injection.