To examine the cytotoxicity of a patient's serum with an acute relapsing sensory neuropathy syndrome, dorsal root ganglion neurons from young adult rats were cultured in the presence of the patient's serum which had an extremely higher-titer monoclonal IgM antibody recognizing B-series gangliosides, GD2, GD1b, GT1b and GQ1b. By the addition of the inactivated patient's serum, the relatively larger cells died after undergoing of metamorphosis during several hours of culture, whilst the smaller cells survived. The IgM fraction isolated from the patient's serum showed similar cytotoxicity towards the neurons as the inactivated whole serum. No cytotoxicity was observed with the IgM fraction-containing medium after it had been absorbed with ganglioside GD1b. The results suggested that the anti-B-series ganglioside-directed antibody is the causal agent for the human neurologic disease.