Retinal S antigen is a potent autoantigen used for the induction of experimental allergic uveoretinitis (EAU). EAU is an organ-specific disease and shows many similarities to other autoimmune diseases such as experimental encephalomyelitis. This paper describes the preparation of highly purified S antigen by using a one-step ion-exchange method. High yields of the protein were obtained. S antigen prepared by this method induces a prolonged posterior uveoretinitis with cellular infiltration in the vitreous and specific loss of retinal photoreceptor cells. The purity of the protein was checked by silver-stained SDS-polyacrylamide gels and immunoblotting techniques.