The pathogenic retina antigen, previously localized by immunofluorescence to the photoreceptor cell layers, was identified as a soluble protein having an estimated molecular weight of 55,000. The antigen was separated from guinea pig retina homogenate by successive fractionation techniques using ultra-centrifugation, ammonium sulfate precipitation, and chromatography. Electrophoretic and immunoelectrophoretic analyses indicated that the antigen was obtained in a highly purified form, and as little as 5 μg mixed with adjuvant produced 100% incidence of uveitis in guinea pigs. Formation of a double precipitin band was observed by immunodiffusion and immunoelectrophoresis on testing both crude and purified preparations against homologous guinea pig antiserum. Although this finding suggested antigenic heterogeneity among molecular components of the antigen, the antigen could not be separated into two components by electrophoresis or chromatography.