IgM class antibodies against the ganglioside GM1 have been found in a subgroup of patients with lower motor neuron syndromes and multifocal motor neuropathies (MMN). The pathogenic relevance of these antibodies is still unclear, but some MMN patients with IgM antibodies against GM1 seem to profit from immunosuppressive therapy. A reliable test for IgM antibodies against GM1 may be useful for identifying these patients. We have assessed the comparability of the ELISA tests used for the determination of IgM against GM1 by sending coded serum samples to nine laboratories. In three samples high-titre IgM antibodies against GM1 were detected by all laboratories. This result was confirmed by dot blot immunodetection and thin-layer chromatography immuno-overlay. Seven samples were read as negative by nearly all laboratories. Major discrepancies between laboratories were noted in the analysis of one sample with results ranging from negative to "high titre".