Bacteria of the genus Proteus are a common cause of urinary tract infections. The O-polysaccharide chain of their LPS (O-antigen) defines the serological specificity of these bacteria. Based on the immunospecificity of the O-antigens, two species, P. mirabilis and P. vulgaris, were classified into 49 O-serogroups, and more O-serogroups for strains of these species and P. penneri have been subsequently proposed.The lipopolysaccharide of P.mirabilis CCUG 19011 from serogroup O19 was degraded under mildly acidic and mildly alkaline conditions. Polysaccharides thus obtained were studied by chemical methods, including O -deacetylation, sugar and methylation analyses, and 1H- and 13C-NMR spectroscopy. Antisera were obtained by immunization of New Zealand white rabbits with heat-killed bacteria. In serological studies, enzyme immunosorbent assay, passive hemolysis test, and inhibition of passive hemolysis were used.The following structure of the O-polysaccharide repeating unit was established:-->3)- beta-D-GlcrhoNAc-(1-->3)- alpha-D-GalrhoNAc4,6(R-Pyr)-(1-->4)- a-D-GalrhoA-(1-->3) alpha-L-Rhap2Ac-(1-->where R-Pyr is (R)-1-carboxyethylidene (an acetal-linked pyruvic acid). This structure is significantly different from the O-polysaccharide structures of P. vulgaris, P.hauseri and P. penneri strains from the same Proteus serogroup O19.Based on immunochemical studies of the lipopolysaccharides, it is suggested 1) to keep P. vulgaris CCUG 4654 and P. penneri 31 in serogroup O19 as two subgroups, 2) to reclassify P. mirabilis CCUG 19011 into a new Proteus serogroup, O51, and 3) to classify serologically related strains, including P. vulgaris ATCC 49990, P. hauseri> 1732-80 and 1086-80, P. penneri 15, and some other P. penneri strains, in yet another Proteus serogroup, O52.