The aim of this work was to serotype Proteus mirabilis urinary tract infection (UTI) strains based on chemically defined O-antigens with the use of two clinical collections from Sweden and Poland consisting of 99 and 24 UTI strains, respectively. A simple two-step serotyping scheme was proposed using enzyme immunoassay with heat-stable surface antigens of Proteus cells and immunoblotting with isolated lipopolysaccharides (LPSs). Using polyclonal anti-P. mirabilis rabbit antisera, 50 Swedish and 8 Polish strains were classified into serogroups O10, O38, O36, O30, O17, O23, O9, O40, O49, O27, O5, O13, O24, O14, and O33. From the Swedish strains, 10 belonged to serogroup O10 and five to each of serogroups O38, O36, and O9. Therefore, none of the O-serogroups was predominant. The majority of the serotyped clinical strains possess acidic O-antigens containing uronic acids and various acidic non-carbohydrate substituents. In immunoblotting, antisera cross-reacted with both O-antigen and core of LPSs. The core region of 19 LPSs bound a single serum, and that of 12 LPSs bound more than two sera. Following bioinformatic analysis of the available sequences, a molecular approach to the prediction of Proteus core oligosaccharide structures was proposed. The identification of the core type of P. mirabilis R110, derived from a serogroup O3 wild strain, using restriction fragments length polymorphism analysis of galacturonic acid transferase is shown as an example. In summary, the most frequent O-serogroups among P. mirabilis UTI stains were identified. The diversity of serological reactions of LPSs is useful for serotyping of P. mirabilis clinical isolates. A possible role of the acidic components of O-antigens in UTI is discussed.