On mild acid degradation of Pseudomonas aeruginosa O:3(a),c, O:3a,d,e and O:3(a),d,f lipopolysaccharides O-specific polysaccharides were isolated. The trisaccharide repeating units of O:3(a),c and O:3a,d,e polysaccharides contained 2-acetamido-2,6-dideoxy-D-galactose and 2,3-(1-acetyl-2-methyl-2-imidazolino-5, 4)-2,3-dideoxy-D-mannuronic acid, which were identified previously as the constituents of P. aeruginosa O:3a,b and O:3a,d O-specific polysaccharides, as well as 2,3-diacetamido-2, 3-dideoxyl-L-guluronic acid, which has never before been found in nature. The last monosaccharide was identified without being isolated in the free state, by means of 13C nuclear magnetic resonance spectroscopy. The structures of O:3(a),c and O:3a,d,e polysaccharides were established by selective hydrogen fluoride solvolysis followed by modification of the trisaccharides obtained and analysis of the 13C nuclear magnetic resonance spectra at each modification stage. The polysaccharides possessed similar structures of repeating units differing from each other in the anomeric configuration of the N-acetylfucosamine residue only: leads to 4)DManImU-(beta 1 leads to 4) LGul(NAc)2U(alpha 1 leads to 3)DFucNAc(beta 1-and leads to 4)DManImU(beta 1 leads to 4)LGul(NAc)2U(alpha 1 leads to 3)DFucNAc(alpha 1-, where DManImU = 2,3-(1-acetyl-2-methyl-2-imidazolino-5,4)-2,3-dideoxy-D-mannuronic acid, LGul(NAc)2U = 2,3-diacetamido-2,3-dideoxy-L-guluronic acid, DFucNAc = 2-acetamido-2,6-dideoxy-D-galactose. The same components were detected in the O:3(a),d,f polysaccharide but not in the same stoichiometric ratio; this polysaccharide possessed no regular structure. The immunochemical study of lipopolysaccharides of all five P. aeruginosa O:3 serotypes showed a definite interrelation between their serological properties and structures of the corresponding O-specific polysaccharides.