A loosely bound lipopolysaccharide-protein complex was extracted from cells of Pseudomonas aeruginosa strain 170015 (O:7ab; Lanyi classification) by saline solution and purified from contaminant nucleic acid by Cetavlon treatment followed by precipitation in an ultracentrifuge. The saline-treated cells were re-extracted with hot aqueous phenol to give firmly bound lipopolysaccharide which was isolated from the phenol layer and purified by ultracentrifugaiton. The identity of both lipopolysaccharide preparations was proved by serological and chemical evidence. Mild acid degradation of the lipopolysaccharide resulted in the splitting off of a lipid component and led to polysaccharide which was purified by gel-filtration on a Sephadex G-50 column. The polysaccharide consisted of N-acetyl-D-fucosamine, N-acetyl-L-fucosamine and D-glucose in the ratio 1:1:1. On the basis of nuclear magnetic resonance spectra, results of methylation analysis and two sequential Smith degradations, the following structure can be assigned to the repeating unit of the polysaccharide: -3)LFucNAc(alpha 1-3)DFucNAc(beta 1-2)DGlc(beta 1-. The polysaccharide did not show serological activity whereas alkali-treated lipopolysaccharide readily sensitised sheep erythrocytes and inhibited the passive haemagglutination reaction with anti-(O:7a,b)serum. Evidence is presented that the oligosaccharide repeating units of the polysaccharide and alkali-treated lipopolysaccharide are indistinguishable. Ps. aeruginosa strain 170016 (O:7a,c) was shown to have the O-specific lipopolysaccharide identical with that from strain 170015. The presented data show that subfactors 7b and 7c in the Lanyi classification of Ps. aeruginosa O-antigens seem to relate to components of the bacterial surface other than lipopolysaccharides.