The incidence of anti-GM1 antibodies in the serum of 104 patients with neurological diseases, 35 patients with non-neurological diseases (NND) and 41 normal controls was determined by enzyme-linked immunosorbent assay (ELISA). Anti-GM1 antibodies were found in 90% of patients presenting with a motor neuropathy (all except one had multifocal conduction blocks). A large proportion (60%) of these patients displayed high antibody titer ranging from 101 to 788. A low incidence of anti-GM1 antibodies was found in the other groups of patients, i.e. 21% of amyotrophic lateral sclerosis (ALS), 26% of other neurological diseases (OND) and 23% of NND. High antibody titers ranging from 106 to 260 were found in two (5%) ALS patients, one (2%) OND patient (myasthenia gravis), and one (3%) NND patient (Waldenstrom's disease). This study shows that high titers of anti-GM1 antibodies are found in a large proportion of patients with motor neuropathy with multifocal conduction blocks. This argues for a possible autoimmune origin of this neuropathy. We suggest that anti-GM1 antibody determination should be included systematically in the evaluation of all patients with motor neuron diseases and predominantly motor neuropathies.