Lipopolysaccharides from 13 strains of Pseudomonas aeruginosa representing seven serotypes of the Habs scheme have been analysed. The lipid A fractions, obtained by mild acid hydrolysis of the lipopolysaccharides, contained phosphorylated glucosamine residues substituted with dodecanoic, hexadecanoic, 2 hydroxydodecanoic, 3-hydroxydecanoic, and 3-hydroxydodecanoic acids (hexadecanoic acid and 2-hydroxdodecanoic acid were absent from one lipid A). Low-molecular-weight solutes released during the mild hydrolyses included 2-keto-3-deoxyoctonic acid, inorganic orthophosphates and pyrophosphates, ethanolamine mono, pyro and triphosphates. For most strains two polysaccharide fractions, one of which appeared to be the common core polysaccharide, were obtained. The major identifiable components and their approximate proportions in the core polysaccharides were glucose (3-4), rhamnose (1), galactosamine (1), alanine (1-1.5), phosphorus (4-6) and heptose (1-2). Rhamnose was absent from one polysaccharide another lacked both rhamnose and alanine but contained glucosamine. Small amounts of various amino sugars found in other core polysaccharides could be associated with the presence of higher-molecular-weight material. Such material was isolated from strain NCIB 8626. The high-molecular-weight polysaccharides obtained from ten strains were probably heterogeneous and consisted mainly of amino compounds, though rhamnose was a major component of four polysaccharades and arabinose was present in another. Fucosamine was the most common amino sugar, but quinovosamine, glucosamine, galactosamine, a possible aminohexuronic acid and unidentified amino compounds were also detected. The results of the survey are discussed in terms of the serological classification of the bacteria and of their sensitivity to EDTA.