The structures of two branched variants of blood group A-active glycolipid, Ac and Ad (Hakomori, S., Stellner, K., and Watanabe, K. (1972) Biochem. Biophys. Res. Commun. 49, 1061-1068), isolated from human type A-erythrocytes, were elucidated. The intact glycolipids and fragments produced by treatment with endo-beta-galactosidase from Escherichia freundii (Fukuda, M. N., and Matsumura, G. (1976) J. Biol. Chem. 251, 6218-6225) were analyzed by methylation. In addition, a specific limited enzyme digestion of Ad-glycolipid was made by incorporating the glycolipid into liposomes before endo-beta-galactosidase digestion. The method preferentially hydrolyzed one of the side chains carrying an A-determinant leaving a core which was recovered from the liposomes. The Ac- and Ad- glycolipids have two branched carbohydrate chains with 12 and 14 sugar residues, respectively, and both have two A-determinants as shown below. (formula see text). The structures of A- and H-active variants isolated from subgroups A1 and A2 have been compared. The results indicate that A variants from A1 or A2 erythrocytes have identical structures, and that H-variants isolated from A2 erythrocytes are identical to those isolated from O erythrocytes. Thus, there is no qualitative structural difference in A-active glycolipids between A1 and A2 erythrocytes.