O-specific polysaccharide was obtained on mild acid degradation of lipopolysaccharide of Proteus vulgaris 5/43 belonging to OX19 (O-variants). It was found to contain D-glucose, 2-acetamido-2-deoxy-D-glucose, and 2-acetamido-2,6-dideoxy-L-glucose (QuiNAc, N-acetyl-L-quinovosamine) in the ratio of about 1:2:1, the last-named sugar being rather uncommon for bacterial antigens. A computer-assisted 13C-NMR-based analysis, methylation analysis, and selective cleavage with anhydrous hydrogen fluoride and dilute hydrochloric acid were applied for structural elucidation of the polysaccharide and the following structure was established:----2)-beta-D-Glcp-(1----6)-alpha-D-GlcpNAc-(1----3)- alpha-L-QuipNAc-(1----3)-beta-D-GlcpNAc-(1----. Serological studies of the O-antigen and oligosaccharides derived therefrom revealed the importance of the trisaccharide fragment beta-D-Glcp-(1----6)-alpha-D-GlcpNAc-(1----3)-alpha-L- QuipNAc in manifesting the antigenic specificity. Serological cross-reactions were demonstrated between lipopolysaccharides of P. vulgaris 5/43, 8/44, and OX19 strains.