Cumulative evidence supports the theory that anti-ganglioside antibodies function in the development of Guillain-Barré syndrome (GBS). Some patients have developed GBS after the administration of monosialoganglioside extracted from bovine brain. To clarify the pathogenesis of GBS associated with and without administration of the monosialoganglioside fraction, we investigated serum antibodies to the minor monosialogangliosides GM1b and GM1 alpha in patients with GBS and in control patients. GM1b and GM1 alpha were recognized specifically by the IgG antibody from the GBS patients. Twelve of 20 GBS patients who had high IgG anti-GM1b antibody titers had a preceding gastrointestinal infection. To evaluate the hypothesis that GM1b could be an immunogen, we determined whether a GM1b epitope was present in Campylobacter jejuni isolated from a patient with GBS associated with anti-GM1b antibody. Immunostaining with the monoclonal anti-GM1b antibody indicated that the lipopolysaccharide of the C. jejuni strain has the GM1b epitope. We speculate that an injection of bovine GM1 fraction that contains GM1b, as well as infection by an agent that bears the GM1b epitope, induces production of the anti-GM1b antibody which functions in the development of GBS in some patients.