Glycophorins, the major sialoglycoproteins of red blood cells in many species, are generally considered to be specific to erythroid cells. Using polyclonal antibodies directed against mouse glycophorin (agp), we have identified a glycoprotein antigenically related to glycophorin on the surface of bovine and rat cultured endothelial cells. Immunoblotting with agp identified a single 60-kDa polypeptide on transfers of SDS/polyacrylamide gels of solubilized confluent endothelial monolayers. In addition, a 60-kDa polypeptide was immunoprecipitated by agp from lysates of 12I-labeled intact endothelial cells. Controls with preimmune serum were negative. This antibody interaction was inhibited by murine erythrocyte ghosts and purified glycophorins. Our past work identified several endothelial surface sialoglycoproteins including a 60-kDa glycoprotein (gp6O) that (i) interacts with albumin, (ii) binds Limax flavus, Ricinus communis, and Triticum vulgare agglutinins but not other lectins, (iii) is sequentially precipitated from '25I-labeled cell lysates by using R. communis agglutinin followed by T. vulgare agglutinin, and (iv) is sensitive to sialidase digestion. Immunoblotting of such precipitates with agp demonstrates that lectins recognize the same glycoprotein, namely gp6O. These results indicate that gp6O, a major endothelial surface sialoglycoprotein, shares antigenic epitope(s) with glycophorin.The highly sialylated, polyanionic glycocalyx of the microvascular endothelium creates a significant permselective barrier that restricts intravascular flow and transvascular transport of the molecular and cellular constituents of the blood. The negative charge of most plasma macromolecules, of all circulating blood cells, and ofthe endothelial glycocalyx ensures significant electrorepulsion between these elements, thereby limiting nonspecific contact between the vascular wall and both the blood cells and the plasma proteins (for details, see ref. 1). Little is known about the glycoproteins that form the endothelial glycocalyx. Recently, we have identified on the endothelial surface a group of sialoglycoproteins (gpl4O, gpl20, gplOO, gp6O, and gp47). Some of these proteins may be involved in a number of important vascular phenomena, including receptor-mediated transcytosis, cellular diapedesis, hemostasis, and blood-borne metastasis (1). The specific binding of albumin to the endothelial surface (2-5) via one of thdse sialoglycoproteins, gp6O (6), as well as two other albumin-binding proteins (7,8), apparently increases capillary wall permselectivity (9) by increasing both charge and volume exclusion within the endothelial glycocalyx (10-12).In contrast to endothelial sialoglycoproteins, the main sialoglycoproteins of erythrocytes, collectively known as glycophorins, have been investigated extensively (for review, see refs. 13 and 14) and have served as a benchmark for the study of transmembrane sialoglycoproteins. They have been isolated from various species and shown to contain up to 70%o (wt/wt) carbohydrate, pr...