O-specific polysaccharide was obtained by mild acid degradation of P. mirabilis 033 lipopolysaccharide (LPS). It was found to contain N-acetyl-D-glucosamine, D-glucuronic acid and N-acetyl-L-fucosamine in a ratio 1:1:1. On the basis of the data obtained from 13C-NMR and methylation analysis, the following structure of repeating unit was established: [formula: see text] Selective removal of the D-GlcA significantly decreased reactivity of 033 O-specific polysaccharide with homologous antiserum. This component was plays an immunodominant role. Cross reactivity between anti-033 serum and disaccharide alfa-L-FucNAc-beta-D-GlcNAc containing P. vulgaris 023 and S. arizonae 059 O-specific polysaccharides was also observed.