S-antigen has been quantitated in bovine and rat retina by electroimmunoassay. The molar ratios S-antigen to rhodopsin in photoreceptor cells were close to 1:1.Immunofluorescence studies show that light induces a shift of S-antigen towards the rod outer segments where it concentrates. Assays indicate that S-antigen becomes largely water insoluble but detergent soluble under these conditions. On basis of previous ultrastructural and present results, this has been interpreted as a light induced binding of S-antigen to the rod outer segment membranes.The data support evidence from literature that S-antigen interacts with (rhod)opsin and we conclude that S-antigen plays a major role in the phototransduction process.