We have produced a monoclonal antibody which stains the Müller cells in the region of the photoreceptors, nerve terminals surrounding the horizontal cells, and nerve terminals in the inner plexiform layer in carp, goldfish, and white perch retinas. Electron microscopy showed that the staining in the outer and inner retina was confined to Müller cells and presynaptic terminals, respectively. In the teleost brain, the antibody stained only the optic tectum and efferent fibers from the stratum album centrale. Biochemical characterization by immunoblotting showed that this monoclonal antibody recognizes an approximately 70-kD molecule in both whole retina and brain homogenates, suggesting that the antibody recognizes an identical molecule. No staining was noted in the spleen or the liver. This monoclonal antibody appears to be specific to a molecule common to the Müller cells and presynaptic terminals in the teleost retina, and although it is present in other parts of the central nervous system, it is confined to the visual pathway.