Six monoclonal antibodies (mAbs) against lipopolysaccharides (LPS) from Bordetella pertussis (P1P3, 60.5), B. parapertussis (PP2, PP6, PPB) and B. bronchiseptica (BRg1) were used to examine the presence of antigenic determinants of LPS on B. bronchiseptica cells. Forty-eight clinical isolates of this Gram-negative bacterium (4 canine, 3 equine, 6 porcine, 4 rabbit and 31 human) were examined. Significant cross-reactivities with the heterologous anti-pertussis and anti-parapertussis mAbs were observed. The isolates also exhibited marked antigenic polymorphism. The 48 isolates could be classified in six immunogroups. Purified LPS preparations extracted from some isolates were analysed by ELISA, thin-layer chromatography, and tricine-SDS-PAGE. The results show that four main types of antigenic polymorphism of B. bronchiseptica LPSs exist: (a) heterogeneity of the core, (b) presence or absence of O-chains, (c) differences in the hinge region between O-chain and core, and (d) differences in interactions of LPS with other cell-surface constituents. Smooth-type LPS molecules, detectable with mAb PP6, were more frequently observed in animal isolates (94%) than in human isolates (52%). Reverse frequencies were found with mAb 60.5 (48% of human isolates, 18% of animal isolates), which is unable to react with long-chain LPSs. This observation could be due to the general absence of some lectin-like receptor, specific to the O-chain, on human bronchoalveolar tissues.