The amino acid sequence of the N-terminal tryptic glycopeptide from a minor human erythrocyte membrane sialoglycoprotein (component D or glycophorin C) was determined by manual sequencing. The glycosylation sites were identified by a new procedure for the detection of the glycosylated derivatives released by Edman degradation. The fragment, comprising 47 residues, was found to contain an average of about 12 O-glycosidically linked oligosaccharides and one asparagine-linked carbohydrate chain. An identical hexapeptide sequence occurring in two regions of the glycopeptide provides evidence that it has developed by an internal gene duplication during evolution. In addition, a part of its structure shows a striking similarity to the sequence of a certain region of the MN and Ss erythrocyte membrane sialoglycoproteins (glycophorins A and B), suggesting that the molecules might be related.