We studied 7 patients with an acute motor axonal Guillain-Barré syndrome (GBS), manifested 5 to 15 days after parenteral injection of a commercial ganglioside preparation given for nonspecific pain syndromes. The serum IgG and IgM antibody response to ganglioside was studied serially and the recognition of epitopes on the peripheral nerves and motor end-plates was examined using biotinylated IgG extracted from the patient's serum. Sera from 8 patients treated with the same ganglioside preparation who did not develop neuropathy and from 25 patients with classic GBS never treated with gangliosides were studied concurrently. All patients with ganglioside-related GBS had a rather severe axonal degeneration, incomplete recovery, and high IgG, but not IgM, antiganglioside antibody titers, ranging from 1:320 to 1:10,240. Seven (28%) of the 25 GBS patients had IgG antibody titers, ranging from 1:160 to 1:10,240. None of the ganglioside-treated patients who did not develop GBS and none of the 50 disease control subjects had IgG GM1 antibodies. Purified IgG from the patients with high GM1 antibodies, but not from the others, recognized epitopes at the nodes of Ranvier and the distal motor nerve terminals at the end-plate. We conclude that exogenous ganglioside injections can be immunogenic, triggering IgG antiganglioside antibodies with specificity for motor nerve terminals. In some patients with axonal GBS such antibodies may be Marchkersor mediators of axonal involvement.