Seroreactivities to Campylobacter jejuni or Campylobacter coli and to the gangliosides GM1 and GD1b were studied by quantitative and immunoglobulin class-specific ELISAs in 42 patients with recent onset of acute Guillain-Barré syndrome (GBS), in 39 patients with positive Campylobacter serology but no neurologic disease, and in 52 healthy blood donors. GBS patients showed positive reactivities to C. jejuni in 57%, 26%, and 2% for IgG, IgA, and IgM, respectively, while blood donors had corresponding values of 6%, 2%, and 4%. IgG, IgA, and IgM reactivities to GM1 were 48%, 78%, and 56% for GBS patients and 4%, 2%, and 4% for blood donors, respectively. Reactivities to GD1b were 77%, 51%, and 15% for the GBS group and 2%, 8%, and 8% for the blood donors, respectively. Patients with positive Campylobacter serology but no neurologic disease also had antibodies to GM1 and GD1b in over half of the cases. Increased titers to C. jejuni closely correlated with increased titers to GM1 and GD1b.