Retinal S-antigen has been found to be localized in a specific photoreceptor cell population of squirrel (Tamias striatus) and rhesus monkey retina. In these cone-rich retinas S-antigen seems only to be present in the rods. Differential immunofluorescent staining of the photoreceptor cells was accomplished by using monospecific rabbit-anti bovine S-antigen antiserum, which exhibits very limited species specificity.In view of its co-localization with rhodopsin in rods and considering data from literature, we suggest that S-antigen is a major component of the phototransduction system in these cells.