Abstract O-Specific polysaccharides, together with oligosaccharide products of their degradation, were isolated by GPC after mild acid delipidation of lipopolysaccharides of Proteus penneri strains 19 and 35. The polysaccharides had the same trisaccharide repeating unit containing one residue each of d-galactose, 2-acetamido-2-deoxy- d-glucose, and 2- acetamido-3-O-[(S)-1- carboxyethyl]-2-deoxy- d-glucose ( N-acetylisomuramic acid). On the basis of 1D and 2D 1H and 13C NMR spectroscopy, including 2D correlation spectroscopy (COSY), rotating-frame NOE spectroscopy (ROESY), and H-detected heteronuclear 1H, 13C multiple-quantum coherence (HMQC), the following structure of the repeating unit was established: The oligosaccharide products formed by cleavage of the glycosidic linkage of GlcNAc represent a chemical trisaccharide repeating unit of the polysaccharide and its oligomer homologs. The ease of hydrolysis of the polysaccharide is associated with the closeness of the glycosyl group and the lactic acid residue in N-acetylisomuramic acid. The polysaccharides studied are structurally related to the O-specific polysaccharides of P. penneri strains 62 and 71 studied by us earlier.