The following structure of the repeating unit of the O-specific polysaccharide of Proteus mirabilis O27, containing amides of D-glucuronic and D-galacturonic acids with L-lysine and L-alanine, respectively, N-acetyl-D-glucosamine and phosphoethanolamine, has been determined: (sequence; see text) The main methods used for structural analysis of the polysaccharide were selective solvolysis with anhydrous hydrogen fluoride and Smith degradation, as well as NMR spectroscopy and mass spectrometry. The immunodominant role of the lateral N-acetyl-D-glucosamine and phosphoethanolamine in manifesting the serological specificity of P. mirabilis O27 has been established.