A neutral O-polysaccharide was obtained by mild acid degradation of the lipopolysaccharide of Proteus mirabilis OC (CCUG 10702) and studied by sugar and methylation analyses and 1 H and 13 C NMR spectroscopy. The following structure of the tetrasaccharide repeating unit of the polysaccharide was established: Based on the unique structure of the O-polysaccharide and serological data, we propose classifying P. mirabilis OC (CCUG 10702) into a new separate Proteus serogroup O75. A weak cross-reaction of O-antiserum against P. mirabilis OC with the lipopolysaccharide of P. mirabilis O49 was accounted for by a similarity in the O-polysaccharide structures.