Five major membrane glycoproteins of the BHK-B4 hamster fibroblast plasma membrane have been identified by binding specific rabbit antibodies to the cell surface and by recovering the detergent solubilized immunocomplexes with Protein A-Sepharose immunoadsorption. These glycoproteins, designated as gp45, gp65, gp95, gp130 and gp140, are exposed at the cell surface since: (i) they were accessible to antibodies in intact viable cells; (ii) they were radioiodinated by the lactoperoxidase-glucose oxidase procedure; and (iii) they were cleaved by proteolytic enzymes in conditions affecting only the cell surface. Among these glycoproteins the gp130 is the predominant component and its exposed portion is characterized by lack of sensitivity to trypsin cleavage. Glycoproteins of different molecular weight, but immunologically related to the major hamster membrane glycoproteins, have been detected at the surface of both rat and mouse fibroblasts.