The lipopolysaccharide (LPS) from eight strains of Yersinia pestis which had been cultured at 28³C appeared to be devoid of an O-antigen when analysed by sodium dodecyl sulfate^polyacrylamide gel electrophoresis. LPS isolated from three of these strains which had been cultured at 37³C also appeared to be devoid of an O-antigen. When the LPS from Y. pestis strain CO92 was purified and analysed by matrix-assisted laser desorption-ionisation time-of-flight mass spectrometry, the observed signals were in the mass range predicted for molecules containing lipid A plus the core oligosaccharide but lacking an O-antigen. The nucleotide sequence of Y. pestis strain CO92 revealed the presence of a putative O-antigen gene cluster. However, frame-shift mutations in the ddhB, gmd, fcl and ushA genes are likely to prevent expression of the O-antigen thus explaining the loss of phenotype. ß 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Microbiological Societies.