A soluble antigen was extracted from the retinas of different mammals and purified by isoelectric focusing in a sucrose density gradient-ampholine column (LKB, Sweden). The isoelectic point of the molecule is about 5.5, its molecular weight about 55,000 and its electrophoretic mobility with reference to albumin 0.7. The S antigen is revealed by one precipitin line in immunodiffusion and immunoelectrophoresis with guinea pig immune sera against bovine or guinea pig retinas, and shows an identity reaction with S antigen from other species. The antigen, inoculated with complete Freund's adjuvant into the guinea pig foot pad, induces experimental uveo-retinitis.