The human continuous leukemia cell line K562 synthesizes and expresses on its surface the major red cell sialoglycoprotein, glycophorin A. Glycophorin A contains 1 N-glycosidic and 15 O-glycosidic oligosaccharides, which are attached to known sites on the polypeptide chain. By immune precipitation with specific anti-glycophorin A antiserum of radioactively labeled cells followed by polyacrylamide gel electrophoresis, we have been able to study its biosynthesis in considerable detail. The synthesis of the N-glycosidic oligosaccharide of glycophorin A is inhibited by the antibiotic tunicamycin, while the O-glycosidic oligosaccharides are not affected. Incomplete glycophorin A, lacking the N-glycosidic oligosaccharide, is apparently incorporated normally into the surface membrane, but the total amount of glycophorin A is decreased. Thus, N-glycosylation is not necessary for externalization of glycophorin A.