We have identified a novel glycoprotein expressed exclusively in frog olfactory neuroepithelium, which we have named "olfactomedin". Olfactomedin is a 57-kDa glycoprotein recognized by seven monoclonal antibodies, previously shown to react solely with proteins of olfactory cilia preparations. It undergoes posttranslational modifications, including dimerization via intermolecular disulfides and attachment of complex carbohydrate moieties that contain N-acetylglucosamine and beta-D-galactoside sugars. Olfactomedin strongly binds to Ricinus communis agglutinin I and has been purified to homogeneity by lectin affinity chromatography. Polyclonal rabbit antiserum raised against purified olfactomedin confirmed that it is expressed only in olfactory tissue. Immunohistochemical studies at the light microscopic and electron microscopic level show that olfactomedin is localized in secretory granules of sustentacular cells, in acinar cells of olfactory glands, and at the mucociliary surface. The massive production of olfactomedin and its striking deposition at the chemosensory surface of the olfactory neuroepithelium suggest a role for this protein in chemoreception.