The phenol phase soluble lipopolysaccharide of Pseudomonas maltophilia strain 555, obtained from cells by the hot aqueous phenol method, was of the smooth type. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, hydrolysis, methylation, and 13C and 1H nuclear magnetic resonance analyses showed that this lipopolysaccharide has an O-chain polysaccharide composed of a repeating pentasaccharide unit, containing D-rhamnose (D-Rha, one part), 3-acetamido-3,6-dideoxy-D-galactose (D-Fuc3NAc, one part), and 4-acetamido-4,6-dideoxy-D-mannose (D-Rha4NAc, three parts) and having the structure (formula; see text) The serological cross-reactions between P. maltophilia 555 and Brucella species can now be related to the occurrence of N-acyl derivatives of 4-amino-4,6-dideoxy-D-mannopyranosyl residues in the O-chains of their respective lipopolysaccharide components.