Anti-GM1 IgM antibodies in motor neuron disease and neuropathy. We found anti-GM1 IgM antibodies in 23% of 56 patients with motor neuron disease (MND), in 19% of 69 patients with neuropathy, and in 7% of 107 controls with other neurologic and nonneurologic diseases. Most of these patients had anti-GM1 IgM antibody titers of 1:80 or less; slightly higher antibody titers (up to 1:640) were found in 3 patients, 1 with MND and 2 with neuropathy, and very high titers (1:20,480) in a patient with MND and an IgM kappa M protein that reacted with GM1, GD1b, and asialo GM1. Six other patients with anti-GM1 IgM that also bound to GD1b. Reactivity with GD1b did not correlate with anti-GM1 titers but was only present in patients with MND or neuropathy. Anti-GM1 IgM antibodies may be a normal constituent of the human antibody repertoire but their frequency and, in some cases, their levels are higher in patients with MND and neuropathy. The origin and the pathogenetic role of these antibodies in neural impairment remain to be established. 