The immunocytochemical localization of S-antigen, a specific protein first discovered in retinal photoreceptors, was studied in the pineal complex of vertebrates (eel, pike, frog, lizard, passerines, mouse, hamster) using monoclonal antibody immunofluorescence. S-antigen immunoreactivity was demonstrated concurrently in retinal photoreceptors and in most pineal phototransducers of all species, i.e. in pineal cells of the receptor series (cone-like, modified photoreceptor cells, pinealocytes) and in cone-like photoreceptors of the frog frontal organ and lizard parietal eye. The labelling was distributed either in all compartments of these cells, or restricted to outer segments. The functional significance of the S-antigen as well as some phylogenetic and ontogenic implication of this marker are discussed.