Bordetella hinzii has recently been isolated from immunocompromised human hosts. The structure of the lipid A of its endotoxin was investigated using chemical analyses, nuclear magnetic resonnance (NMR), gas liquid chromatography/mass spectrometry (GC/MS), plasma desorption mass spectrometry (PDMS) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The lipid A contains the classical bisphosphorylated b-(1β6)-linked D-glucosamine disaccharide with hydroxytetradecanoic acid (C 14 OH) in amide linkages. The lipid A components of B. pertussis, B. bronchiseptica, and B. parapertussis all differ in their acylation pattern but share a residue of tetradecanoyl-3-hydroxytetradecanoic acid in amide linkage at the C-2' position. However, in the B. hinzii species, the tetradecanoic acid (C 14 ) is stoichiometrically replaced by a 2-hydroxytetradecanoic acid (2-C 14 OH). In the few reported examples of a hydroxylated fatty acid in this position, the substitutions were only partial. The B. hinzii lipid A differs from that of B. pertussis also by replacement of the hydroxydecanoic acid (C 10 OH) by hydroxydodecanoic acid (C 12 OH) and by the presence of a hexadecanoic acid (C 16 ) to give a sixth fatty acid. The lipid A was heterogeneous, being composed of three major molecular species: tetra-, penta-and hexaacylated. The fatty acids in ester linkage were localized by PDMS of the native and alkali-treated lipid A. The lipid A components isolated from the Ochain-linked lipopolysaccharides (LPSs) were shown to be more acylated than those from the O-chain-free LPSs.