The structure of branched lactosaminoglycan isolated from Band 3 glycoprotein of adult human erythrocytes was elucidated. The glycopeptides were digested by endo-beta-galactosidase under various conditions and oligosaccharides and core glycopeptides thus obtained and intact glycopeptides were analyzed by methylation, exoglycosidase digestion, and fast atom bombardment mass spectrometry. Based on these experiments, the structure of adult lactosaminoglycan was found to have the following unique features: 1) two (as major components) or three (as minor components) polylactosaminyl side chains composed of (Gal beta 1----4 GlcNAc beta 1----3) repeating units are attached to the core portion, the structure of which is Man alpha 1----6(Man alpha 1----3) [+/- GlcNAc beta 1----4]Man beta 1----4GlcNAc beta 1----4(Fuc alpha 1----6)GlcNAc----Asn, 2) the polylactosaminyl side chain arising from the mannose C-6 side is composed of 10-12 N-acetyllactosaminyl units and three branches; the lactosaminyl side chain arising from the mannose C-3 side is composed of 5-6 lactosaminyl units and contains one or two branches, 3) each branch is short and is composed of the Gal beta 1----4GlcNAc beta 1----6 structure, 4) fucose and sialic acid are preferentially linked to nonreducing terminal regions but not on the branches in the internal portion of the polylactosaminyl chain.