Guillain–Barré syndrome (GBS), an autoimmune disease of the peripheral nervous system, is associated with antecedent Campylobacter jejuni infection. GM and KM allotypes—genetic markers of immunoglobulin γ and κ chains, respectively—are implicated in the etiopathogenesis of several autoimmune diseases. To determine if GM/KM phenotypes are associated with GBS and influence antibody responses to C. jejuni and to GM1 and GD1a gangliosides, 72 Japanese GBS patients and 73 controls were allotyped for several GM and KM markers. Sera from patients were characterized for antibodies to C. jejuni, GM1, and GD1a. The distribution of KM phenotypes was significantly different in patients with anti-GD1a ganglioside antibodies from those who lacked these antibodies (P=0.029). No other significant associations were found. These results suggest that KM allotypes are not risk factors for developing GBS, but contribute significantly to the generation of autoimmune responses to GD1a ganglioside in patients with this disease.