Experimental autoimmune uveoretinitis (EAU) induced by immunization of albino Lewis rats with a retinal soluble antigen (S-antigen) has been studied extensively by many workers. An intraorchidic injection of S-antigen 4 days prior to immunization of the animal with the antigen emulsified in adjuvant was found to prevent the onset of the disease. Orchiectomy in 24 hr after the intraorchidic injection did not abolish the effect of treatment. The systemic suppression induced by the orchidic treatment persisted at least for 3 weeks after treatment. In rats that received orchidic treatment, delayed-type hypersensitivity was markedly inhibited but anti-S-antigen antibody levels in the serum were as high as those in immunized rats without orchidic pretreat-ment. These results indicate that antigen challenge to the testis is a novel method to elicit systemic activation of the immunosuppressive mechanism.