The distribution in mouse retina of a 33,000 Da phosphoprotein (33 kDa) that complexes with the beta/gamma subunits of transducin (T beta gamma) and undergoes light-induced dephosphorylation was determined by immunocytochemistry. An antiserum containing antibodies for the 33 kDa protein and beta-transducin of mouse and bovine retinas was generated against the purified 33 kDa-T beta gamma complex from bovine retina. The antiserum reacts with beta-transducin derived from either 33 kDa-T beta gamma complex or transducin complex (T alpha beta gamma), but not with the alpha- or gamma-transducin. It also reacts with both the phosphorylated and unphosphorylated form of the 33 kDa-T beta gamma complex. Antibodies, monospecific for the 33 kDa and beta-transducin subunits respectively, were purified from the antiserum by immunoadsorption and used in immunocytochemical analysis of the respective antigens. The 33 kDa protein was found to be associated exclusively with the photoreceptor cells of the retinas, with the most intense staining in the inner and outer segments' layers and lighter staining in the synaptic terminal layers. beta-Transducin also is found in the photoreceptors, but some T beta immunoreactivity exists within the inner plexiform layer. The specific localization of the 33 kDa protein together with its light-modulated phosphorylation suggest that the 33 kDa-T beta gamma complex is involved in light-regulated activities of the rod photoreceptor cells.