Immunization of albino LEW rats with a retinal soluble antigen (S-antigen) induces experimental autoimmune uveoretinitis (EAU) which shows clinical features resembling those of human uveitis. Several uveitogenic epitopes have been identified in the antigen. This study reports that an intratesticular injection of low doses of a uveitogenic eicosapeptide (P343-362) of S-antigen prior to immunization with the same peptide prevented the onset of EAU by inducing systemic tolerance, designated orchidic tolerance. Splenic lymphocytes of both CD4+and CD8+subsets from tolerized rats transferred orchidic tolerance to syngeneic recipients and protected them from subsequent EAU induction. Orchidic tolerance elicited by low antigen dosage was mediated, in part, by active suppression due to suppressor or regulatory cells. At high antigen doses, however, regulatory activity was reduced possibly due to the induction of anergy in regulatory cells, and EAU severity increased. The CD4+regulatory T cells from tolerized rats showed enhanced expression of IL-4 mRNA compared with CD4+cells from control rats. Increased immunoreactivity for IL-4, IL-10 and TGF-βwas observed in the spleen and lymph nodes of tolerized animals. The results suggest that orchidic tolerance induced by low doses of P343–362 is mediated in part by CD4+regulatory cells secreting Th2 cytokines.