Increased titers of antibodies to GM1 ganglioside in humans are associated with lower motor neuron disease and predominantly motor neuropathy with or without conduction block. To investigate the possible mechanism of these antibodies, we injected the serum of a patient with anti-GM1 antibodies who had motor neuron disease and multifocal motor conduction block, into rat sciatic nerve. When injected with fresh human complement, the serum-induced conduction block with temporal dispersion and deposits of immunoglobulin were detected at the nodes of Ranvier. Electron microscopic studies revealed demyelination in 6.5% of the fibers. After preabsorption with GM1, the serum had no effect, suggesting that the anti-GM1 antibodies were responsible for the conduction abnormalities.