The cell-surface lipopolysaccharide produced byMoraxellacatarrhalisserotype A is composed of a hydrophobic lipid A moiety and an oligosaccharide, but lacks high-molecular-weight O-polysaccharide chains. The oligosaccharide component is composed ofD-glucose,D-galactose,D-glucosamine, and 3-deoxy-D-manno-octulosonic acid. The carbohydrate backbone was obtained from the lipopolysaccharide by employing a reaction sequence involving deacylation, dephosphorylation, and reduction of the reducingD-glucosamine terminus of the lipid A moiety. Structural analysis of the backbone oligosaccharide employed a combination of microanalytical methods and nuclear magnetic resonance spectroscopy. Homo- and hetero-nuclear chemical shift correlation techniques and nuclear Overhauser enhancement (NOE) experiments led to the unambiguous assignment of the1H andl3C resonances associated with each of the component glycosyl residues and established their sequence within the backbone oligosaccharide as shown,This lipopolysaccharide was found to consist of a highly branched,D-glucose-containing inner-core region which possesses a unique and unusual solution conformation. This was established by measurement of transglycosidic NOEs and three-bond1H−13C coupling constants, in conjunction with molecular modeling. AD-galactose-containing disaccharide identified as a terminal group of the lipopolysaccharide was structurally identical to antigenic epitopes expressed by certain mammalian epithelial cells and may be related to the virulence potential of this human pathogen.