Immune blots of electrophoresed lipopolysaccharides extracted from 38 Campylobacter jejuni serostrains suggested the presence of O chains in 16 strains and their absence in 22. Structural analysis confirmed the presence of O chains in serostrains O:19, O:23, and O:36 and the absence of O chains in serostrains O:1, O:2, and O:3. The O:19 strain has O repeat units of beta-D-glucuronic acid amidated with 2-amino-2-deoxyglycerol and N-acetylglucosamine. The 0:36 O chain has four different but closely related repeat units that each consist of N-acetylglucosamine, galactose, and a heptose that is varied in structure from one repeat unit to the next. The O:23 O chain has three different repeat units identical to three of O:36. Except for O:3, core oligosaccharides of strains with or without O chains contain sialic acid (Neu5Ac) in the terminal regions that in many cases mimic the structures of human gangliosides. Three neuropathic isolates were found to have a core terminal trisaccharide (Neu5Ac alpha2-->8Neu5Ac alpha2-->3Galbeta1) that was not found in nonneuropathic strains.