The O-specific polysaccharide chain (O-antigen) of Proteus penneri strain 22 lipopolysaccharide was studied using chemical methods, including partial acid hydrolysis and Smith degradation, as well as one- and two-dimensional 1H and 13C NMR spectroscopy. The following structure of the pentasaccharide repeating unit was established: [sequence: see text] The O-specific polysaccharide contains a GalNAc residue in the furanose form which has not been hitherto found in bacterial polysaccharides. The O-antigen studied is serologically and structurally unique among Proteus strains and, therefore, a new Proteus serogroup O63 is proposed for P. penneri strain 22.