Sera of patients who develop Guillain-Barré syndrome (GBS) subsequent to Campylobacter jejuni enteritis frequently have IgG anti-GM1 antibody. Lipopolysaccharide (LPS) of C. jejuni isolated from a GBS patient has a GM1 ganglioside-like structure. IgG subclass distribution of the anti-GM1 antibody in GBS patients is mainly restricted to IgG1 and IgG3. Since IgG antibodies to bacterial polysaccharide generally are restricted to IgG2 subclass, some investigators have assumed that either the general rules for immune response to LPS are broken in the patients or an alternative antigen has yet to be identified. To clarify whether the LPS participates in the production of the anti-GM1 antibody, we investigated the subclass of IgG antibody to the LPS that bears GM1-like structure. The subclasses of IgG antibody to the LPS were restricted predominantly to IgG1 and IgG3. The GM1 epitope-bearing LPS may function in the production of the anti-GM1 antibody in patients with GBS subsequent to C. jejuni infection.