The major human erythrocyte membrane (MN-) sialoglycoprotein was purified from MM, MN and NN cells using detergent gel and ion exchange chromatography. N-terminal analyses with dansyl-chloride revealed serine in preparations from MM and leucine in those from NN erythrocytes, whereas glycoprotein isolated from MN cells contained both the above amino acids. These data strongly suggest that the above residues may represent the structural difference between the M and N antigens. Evidence was also obtained that the Ss-glycoprotein, which is associated with "N" activity, exhibits the same N-terminal amino acid (leucine) as the MN glycoprotein from NN cells.