The O-polysaccharide of the lipopolysaccharide produced by Salmonella milwaukee O:43 (group U) was shown by composition analysis, methylation, periodate oxidation and 1H and 13C nuclear magnetic resonance spectroscopic analytical methods to be a polymer of branched pentasaccharide repeating units having the structure:[Formula: see text]The blood-group activity of the O-polysaccharide was established by its serological reactivity with a specific monoclonal antibody to human blood group B, using passive hemaglutination and ELISA assays, indicating the common antigenic epitope to be a nonreducing terminal trisaccharide unit composed of L-Fucp and D-Galp (1:2) residues.Key words: Salmonella milwaukee, lipopolysaccharide, blood group B, polysaccharide, nuclear magnetic resonance.