The localization of S-antigen, the soluble, uveitogenic, 51 kDa protein of the retina has been studied. Highly specific rabbit anti-bovine S-antigen antiserum reacted predominantly with the outer segment layer of the photoreceptor cells of rat retina in the indirect immunofluorescence technique, provided that the tissue was fixed by perfusion of the light adapted animal.More detailed information was obtained by immuno-electron microscopy using the same fixation technique and Protein A-coated gold particles for labeling. S-antigen was found to be predominantly bound to the rod outer segment plasma membranes and to the discs. The possible implication of this localization is discussed in view of the function of S-antigen.