Increased titers of IgG anti-GM1 and anti-asialo GM1 (GA1) ganglioside antibodies are present in some patients with the Guillain-Barré syndrome, particularly with the motor axonal variant, and following infection with Campylobacter jejuni or parenteral administration of gangliosides. The subclass distribution of IgG anti-GM1 or GA1 antibodies from 19 patients with acute motor neuropathy and elevated antibody titers were measured by ELISA using mouse monoclonal antibodies specific for human IgG subclasses. The anti-GM1 or GA1 antibodies were predominantly of the IgG1 and IgG3 subclasses, which are capable of complement fixation, and are characteristic of a T cell-dependent antibody response.