An O-specific polysaccharide was isolated by mild acid degradation of the lipopolysaccharide of Proteus vulgaris O45 and studied by sugar and methylation analyses along with 1 H and 1 3 C NMR spectroscopy, including 2D COSY, TOCSY, ROESY, H-detected 1 H, 1 3 C HSQC and HMBC experiments. The following structure of the pentasaccharide repeating unit of the polysaccharide was established:->6)-α-D-GlcpNAc-(1->4)-α-D-GalpNAc-(1->4)-α-D-GalpA-(1->3)-@ b-D-GlcpNAc-(1->2)-β-D-Fucp3NAc4Ac-(1->where Fuc3NAc4Ac is 3-acetamido-4-O-acetyl-3,6-dideoxygalactose. A cross-reactivity of anti-P. vulgaris O45 serum was observed with several other Proteus lipopolysaccharides, which contains Fuc3N derivatives.