Differences in amino acid sequence of erythrocyte membrane glycophorin A are correlated with M or N blood group activity. A second sialoglycoprotein, glycophorin B, has an amino acid sequence identical to that of glycophorin AN in the first 23 positions and carries N activity only, suggesting that different structural genes code for the glycoproteins carrying these antigens. Certain genetically variant cells lack glycophorin A, as determined by immunochemical methods, and serological MN activity. Other variants lack MN activity, but contain normal amounts of glycophorin A in the membrane.