The structure of the oligosaccharide part of the Haemophilus influenzae RM.118-28 lipopolysaccharide (LPS) has been investigated. The oligosaccharide was obtained from the LPS by mild acid hydrolysis followed by gel-permeation chromatography, and was studied by methylation analysis, NMR spectroscopy and mass spectrometry. The structure of the major compound, which is a hexasaccharide, is proposed as follows. [formula: see text] In the structure, Kdo is 3-deoxy-D-manno-octulosonic acid, PEtn is phosphoethanolamine, PCho is phosphocholine and L,D-Hep is L-glycero-D-manno-heptose. Electrospray-ionization mass spectrometry on O-deacylated LPS obtained after treatment with anhydrous hydrazine gave evidence for the presence of two minor compounds, which show additional substitution of the main structure with phosphate and PEtn, respectively. These substitutions have not been localized.