A new monoclonal antibody (termed WH-1; isotype IgG2b) was established using a homogenate of dissected guinea pig cochleas (N = 60) as immunogen. Western blotting and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) identified the WH-1 antigen as a protein or glycoprotein with M(r) approximately 40 kDa. Immunoperoxidase treatment of histologic cryosections of guinea pig cochlea, followed by light microscopic examination, revealed strong positive staining at three sites: (i) parts of Hensen's stripe, marginal band, covering net, and Kimura's membrane (within the tectorial membrane [TM]); (ii) Deiters' cells, pillar cells, Hensen's cells, and stereocilia of outer hair cells (within the organ of Corti); (iii) interdental cells, inner and outer sulcus cells, Reissner's membrane, and surface membrane of stria vascularis epithelium. Similar staining patterns were observed for cryosections of rat and mouse cochleas. Only a trace quantity of cross-reacting protein was detectable in brainstem. The protein was not detectable in tongue extract by Western blotting. However, sections of brainstem and tongue did show positive immunohistological staining with WH-1. Localization of WH-1 antigen was further examined by electron microscopy. WH-1 positivity on outer hair cell stereocilia, certain sites on the TM, interdental cell surface, Reissner's membrane epithelia, and inner and outer sulcus cells was confirmed. WH-1 antigen was not detected on inner hair cell stereocilia by light or electron microscopy. The localization of WH-1 antigen on outer hair cell stereocilia and TM suggests that it may play some role in adhesion between these structures.