The structure of the core region of the lipopolysaccharide (LPS) from the nontypable Haemophilus influenzae strain SB 33 was elucidated. The LPS was subjected to a variety of degradative procedures. The structures of the derived oligosaccharide products were established by monosaccharide and methylation analyses, NMR spectroscopy and mass spectrometry. These analyses revealed a series of related phosphocholine (PCho) containing structures differing in the number of hexose residues. The results pointed to each species containing a conserved phosphoethanolamine (PEtn) substituted heptose-containing trisaccharide inner-core moiety. The major LPS glycoforms were identified as 2-Hex, 3-Hex and 4-Hex species according to the number of hexose residues present.