We have demonstrated that varying degrees of seminiferous epithelial damage up to and including aspermatogenesis could be induced by transfer of immune serum. Aspermatogenesis was consistently produced when the recipient guinea pigs were pretreated with Freund’s complete adjuvant plus saline intradermally 7 days prior to transfer of serum from donor guinea pigs immunized 7 days previously with testicular homogenate in Freund’s complete adjuvant. Aspermatogenesis was also induced by transfer of cells of lymph node, spleen or peritoneal exudate from the same immune donor animals.