The chemical structure of lipid A from the lipopolysaccha-ride of the mushroom-associated bacterium Pseudomonas reactans,a pathogen of cultivated mushroom, was elucida-ted by compositional analysis and spectroscopic methods (MALDI-TOF and two-dimensional NMR). The sugar backbone was composed of theb-(1¢fi6)-linkedD-gluco-samine disaccharide 1-phosphate.