Lipopolysaccharide was isolated from dry biomass of the Pseudomonas fluorescens strain IMV 472 (biovar I) by the Westphal procedure and purified by ultracentrifugation. Fractions of the structural components of the lipopolysaccharide macromolecule-lipid A, the core oligosaccharide, and the O-chain-were isolated and characterized. 3-Hydroxydecanoic, 2-hydroxydodecanoic, 3-hydroxydodecanoic, hexadecenoic, octadecenoic, and hexadecanoic acids were identified as the main fatty acids of lipid A. The hydrophilic moiety of lipid A was found to include glucosamine, phosphoethanolamine, and phosphate. Glucose, galactose, rhamnose, glucosamine, galactosamine, alanine, phosphoethanolamine, and 2-keto-3-deoxyoctulosonic were identified in the core region of lipid A. The O-specific polysaccharide chain consisted of a repeating tetrasaccharide fragment that included two L-rhamnose residues, a D-fucose residue, and an N-acetyl-D-glucosamine residue as a side substituent. Double immunodiffusion in agar and the lipopolysaccharide precipitation reaction revealed no serological interrelation between strain IMV 472 and P. fluorescens strains studied previously.