Monoclonal antibodies specific for the retinal S-antigen were obtained by hybridization of spleen cells from a BALB/c mouse immunized with bovine S-antigen and NS-1 myeloma cells. Five selected antibodies specifically labeled the photoreceptor cells of the retina by immunofluorescence. Whereas antibody S9E2 only reacted with bovine S-antigen, the other antibodies showed interspecies cross-reactivity. They were used for the characterization of specific epitopes of S-antigen in photoreceptors from a wide range of species representative of various classes of vertebrates and invertebrates. The presence of S-antigen in distant species (vertebrates, Amphioxus, nemerteans, annelids, molluscs) indicates a high phylogenetic stability and suggests an important role for this protein in photoreceptor function.