A new blood group A-active glycolipid fraction, termed Ax, showing a chromatographic mobility between Aa and Ab was found in blood group A1 erythrocytes but not in A2 erythrocytes. Ax was identified by its conversion to "globo H" by alpha-N-acetylgalactosaminidase and by 1H-NMR spectroscopy as GalNAc alpha l----3[Fuc alpha l----2]Gal beta l----3GalNAc beta l----3Gal alpha l----4Gal beta l----4Glc beta l----lCer. Globo-H (Fuc alpha l----2Gal beta l----3GalNac beta l----3Gal alpha l----4Gal beta l----4Glc beta l----lCer) was found in blood group A, and O but not in A1 erythrocytes. Thus, one of the A1-specific determinants must be an A determinant carried by globo-series structure.