Lipopolysaccharides from Pseudomonas aeruginosa seem to have the same general architecture as those from enterobacteria. Points of difference include the fatty acid composition and the unusually high degree of phosphorylation (triphosphate residues have been detected). The outer region of the core oligosaccharides is also distinctive; its components are n-glucose, l-rhamnose, n-galactosamine, and possibly l-alanine. Lipopolysaccharide preparations contain a relatively low proportion of complete (S-form) molecules, but in some cases contain a significant proportion of molecules that have a single O-specific repeating unit attached to the core oligosaccharide. The O-specific side chains are typically rich in amino sugars, including novel types. Components identified so far are 2-amino-2-deoxyglucose, 2-amino-2-deoxygalactose, 2-amino-2,6-dideoxyglucose, 2-amino-2,6-dideoxygalactose, 2-amino-2-deoxygalacturonic acid, 2,4-diamino-2,4,6-trideoxyglucose, 2,3-diamino-2,3-dideoxyhexuronic acids (with the d-gluco-, d-manno-, and l-gulo- configurations), and 2,3-(2-methyl-2-imidazolino-5,4)-2,3-dideoxymannuronic acid. In some cases, polymeric material enriched in neutral sugars can be isolated from the O-specific fractions of partly degraded polysaccharides.