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The characterization of a human fibroblast surface glycoprotein, visualized by crossed immunoelectrophoresis using rabbit antibodies against whole fibroblasts, is described. The antigen is synthesized by fibroblasts in culture and was localized both intracellularly and at the cell surface. It was highly antigenic and was detected only in human cells of mesenchymal origin. The glycoprotein occurred in two different forms with alpha 2 and beta electrophoretic mobility. The slow migrating amphiphilic beta form was localized at the cell surface and showed a single protein band with an apparent molecular weight of 150 000 in SDS-polyacrylamide gel electrophoresis. By external papain treatment of intact viable cells, a water-soluble molecule was released with a reduced molecular weight (140 000) and an increased electrophoretic mobility as compared to the native membrane component. This hydrophilic form was also present intracellularly in fibroblasts not treated with exogeneous proteases. The observation that the detergent-solubilized beta form was irreversibly converted to a more anodic form by incubation of whole cell extract at acidic pH, suggested that the intracellular protein represented a lysosomal degradation product of native internalized fibroblast surface glycoprotein.