An antibody directed against protein kinase C (PKC) was applied to various mammalian retinae. In the cat, rat, rabbit, and macaque monkey we found PKC-like immunoreactivity in bipolar cells which had the morphology of rod bipolar cells; in the rat some amacrine cells were also immunoreactive. In the outer plexiform layer, labeled dendrites were always the central elements of the rod spherule invagination, and in the inner plexiform layer only rod bipolar axons and their axon terminals were immunoreactive. The antibody against PKC thus can be used to distinguish rod bipolar cells from cone bipolar cells. The antibody against PKC was used to determine the densities of rods and rod bipolar cells in the cat retina. In the central retina we found a rod to rod bipolar ratio of 16 to 1, in the periphery the ratio increases to 25 to 1. In freshly dissociated retina, cells with rod bipolar morphology could be identified; these cells were also labeled with the anti-PKC antibody. Hence, PKC-like immunoreactivity can be used to recognize rod bipolar cells in vitro.