Representative strains of Bordetella bronchiseptica and B. parapertussis were found to produce smooth lipopolysaccharides (LPS) having identical antigenic O-polysaccharide components composed of linear unbranched polymers of 1,4-linked 2,3-diacetamido-2,3-dideoxy-alpha-L-galacto-pyranosyluronic acid residues. These LPSs differed from the LPS of B. pertussis which produces only rough-type LPS, devoid of O-polysaccharide. While B. bronchiseptica and B. parapertussis had chemically and immunologically identical O-polysaccharide structures, their core oligosaccharide components differed. The core oligosaccharide of B. parapertussis was chemically distinct from the core of B. bronchiseptica which appeared to be structurally and immunologically similar to a core oligosaccharide of B. pertussis.