Retinal S-antigen, a soluble protein present in the photosensory layer of the retina, has been shown to be involved in the pathogenesis of some uveitis. The procedure for the isolation of S-antigen has been continually improved and perfected. In light of the method of A1-Mahdawi and coworkers, with some modification, we obtained good yields of highly purified S-antigen (0.325 mg per retina) from bovine retinas. Experimental auto-immune uveitis (EAU) was successfully induced by injecting the S-antigen into the hind footpad of guinea pigs. The EAU was characterized by lymphocyte infiltration in the pars plana and vitreous base, a counterpart of human intermediate uveitis.