A monoclonal antibody (MAb-1E7), generated against bovine retinal homogenate, labeled the outer and inner segment layers of the vertebrate retina. Immuno-electron microscopic observation clearly demonstrated that antigen(s) bound by MAb-1E7 was localized in the cell membrane of the outer segment and the distal portion of the inner segment. Western blot analysis revealed that MAb-1E7 recognized 40 kD- and 27 kD-polypeptides. Mouse retina with hereditary photoreceptor degeneration (C3H/He and CBA strains) did not involve the MAb-1E7 immunoreactive structures. The present immunocytochemical observation demonstrated that MAb-1E7 was highly specific to the outer segment of the photoreceptor cells and, therefore, can be a useful marker for the cells.