Campylobacter jejuni (Cj) enteritis is the most frequently recognised infection preceding Guillain-Barre syndrome (GBS) and this combination is commonly associated with anti-GM1 ganglioside (anti-GM1) antibodies. We have examined the hypothesis that the anti-GM1 antibodies represent an immune response against the Cj lipopolysaccharide (LPS). We prepared the LPS fraction from 8 isolates of Cj, 3 from GBS patients with acute inflammatory demyelinating polyradiculoneuropathy (AIDP), 3 from patients with Miller Fisher syndrome (MFS) and 2 from enteritis patients without neurological disease. We looked for IgG antibodies against LPS and GM1 in the serum of 10 GBS and 10 MFS patients, including the patients from whom the Cj had been isolated, and 11 normal control subjects. The highest levels of IgG binding to LPS fractions were found in the GBS patient sera and were with one of the LPS fractions extracted from the C. jejuni isolated from a GBS patient, one from a MFS patient and two Cj isolates from enteritis patients without neurological disease. The level of IgG binding to these LPS fractions was related to the level of IgG anti-GM1 antibody in the serum. Affinity-purified anti-GM1 antibodies showed the same pattern of differential binding to the LPS fractions as the serum from which they were derived. Cholera toxin bound to the same LPS fractions as GBS patients' IgG, the binding of which was blocked by the toxin indicating specific antibody reactivity with a GM1 hapten. The presence of serum anti-GM1 antibodies did not coincide with the presence of the GM1 hapten on the LPS of the infecting strain of Campylobacter indicating that anti-GM1 antibodies do not necessarily arise as part of a simple immune response against the LPS. The IgG antibodies binding to LPS were predominantly of the IgG2 isotype but patients with anti-GM1 IgG had mainly antibodies of IgG1 subclass against both LPS and GM1, implying their production by a T-cell dependent mechanism.