Antigenic phenol-phase soluble lipopolysaccharide isolated from Brucella abortus 1119-3 by hot phenol-water extraction was shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, controlled hydrolysis, periodate oxidation, methylation, and 1H and 13C nuclear magnetic resonance studies to be an S-type lipopolysaccharide which could be cleaved to yield a lipid A and an O-chain polysaccharide identified as an unbranched linear homopolymer of 1,2-linked 4,6-dideoxy-4-formamido-alpha-D-mannopyranosyl residues. The serological reactivity of bovine antiserum to B. abortus 1119-3 with the lipopolysaccharides of Yersinia enterocolitica serotype O:9 and Vibrio cholerae species has now been related to the occurrence of 1,2-linked N-acylated 4-amino-4,6-dideoxy-alpha-D-mannopyranosyl units in the O-chain polysaccharides of their lipopolysaccharides.