A novel core-lipid A backbone oligosaccharide was isolated and identified from the lipopolysaccharide fraction of the mushrooms pathogen bacterium Pseudomonas tolaasii. The oligosaccharide was obtained by alkaline treatment of the lipopolysaccharide fraction. Since the repeating unit of the O-antigen contained one residue of -->4)-alpha-l-GulpNAcAN, the hydrolysis was accompanied by beta-elimination on this residue and following depolymerization, producing a mixture of oligosaccharides. The complete structural elucidation showed the presence of a single core glycoform and was achieved by chemical analysis and by (1)H, (31)P, and (13)C NMR spectroscopy applying various 1D and 2D experiments. [structure: see text]. All sugars are alpha-d-pyranoses, if not stated otherwise. Hep is l-glycero-d-manno-heptose, Kdo is 3-deoxy-d-manno-oct-2-ulosonic acid, P is phosphate. QuiN and DeltaGulNA are present in nonstoichiometric amount.