Introduction: Bacteria of the genus Proteus are a common cause of urinary tract infections. The O-polysaccharide (OPS) chain of their lipopolysaccharide (LPS) defines the serological specificity of strains. Based on the OPS structures and the immunospecificity of the LPS, Proteus strains have been classified into 74 O-serogroups. Materials and Methods: The OPS of P. mirabilis TG 115 was obtained by mild acid degradation of the LPS and studied by 1H and 13C nuclear magnetic resonance spectroscopy. Antisera were raised by immunization of rabbits with heat-killed bacteria. Serological studies were performed using enzyme immunosorbent assay, passive immunohemolysis, inhibition experiments, absorption of O-antisera, and Western blot.Results: The following structure of the P. mirabilis TG 115 OPS was established: 2)--D-GalpA-(13)--D-GalpNAc-(14)--D-GalpA-(13)--D-GlcpNAc-(1 The same structure has been reported previously for the O-polysaccharides of P. mirabilis CCUG 10701 (O74) and P. mirabilis 41/57 (O23), except that they contain O-acetyl groups in non-stoichiometric quantities. Serological studies showed the antigenic identity of the three strains and their close serological relatedness to P. vulgaris 44/57. Conclusions: Based on the OPS structures and serological data, it is suggested to classify P. mirabilis 41/57, TG 115, and CCUG 10701 into one subgroup and P. mirabilis 42/57 and P. vulgaris 43/57 and 44/57 into another subgroup of the Proteus O23 serogroup.