A neutral O-specific polysaccharide was obtained by mild acid hydrolysis of the lipopolysaccharide (LPS) of Salmonella Abortusequi O4 bacterium (previously serogroup B). As determined by compositional analyses and NMR spectroscopy, the O-polysaccharide consists of four or five residues in the repeating subunit. The assigned structures are: alpha-Abep -->2)-alpha-D-Manp-(1-->4)-alpha-L-Rhap-(1-->3)-alpha-D-Galp-(1--> and alpha-Abep alpha-D-Glcp -->2)-alpha-D-Manp-(1-->4)-alpha-L-Rhap-(1-->3)-alpha-D-Galp-(1-->. A distribution of the repeating units in O-chain was analysed by Western blotting with anti O12 serum and MS spectrometry of oligosaccharides obtained from partial hydrolysis of polysaccharide.