The purified high molecular weight serogroup Y meningococcal polysaccharide contains equimolar proportions of D-glucose and N-acetylneuraminic acid and is partially O-acetylated. Carbon-13 nuclear magnetic resonance (NMR) studies, together with other chemical data, have indicated that the polysaccharide is linked only at C-6 of the D-glucose and C-4 of the sialic acid residues, all the linkages being in the alpha-configuration. The 13CNMR data also indicated that the Y polysaccharide is composed of an alternating sequence of these two different residues, and this was confirmed by its autohydrolysis where the major product was 4-O-alpha-D-glucopyranosyl-beta-D-N-acetylneuraminic acid. The W-135 polysaccharide differs from that of Y only in the absence of O-acetylation and in the configuration of one hydroxyl group of the disaccharide repeating unit. In this case autohydrolysis yielded 4-O-alpha-D-galactopyranosyl-beta-D-N-acetylneuraminic acid as the major product. Structural evidence indicates that the BO and Y polysaccharides are identical. Methanolysis of the Y polysaccharide yielded in addition to the methyl glycosides of glucose and sialic acid, a 9-O-acetyl derivative of the latter. This derivative was formed during the re-N-acetylation process and its formation was mainly due to the presence of sodium ions in the original polysaccharide.