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Guillain-Barre syndrome with antibodies against ganglioside N-acetylgalactosaminyl GD1a (GalNAc-GD1a) is characterized by a rapid onset of predominantly distal pure motor neuropathy. However, the pathogenic role of anti- GalNAc-GD1a antibody and calcium channels in neuromuscular junctions (NMJs) remains unclear. We investigated the effects of IgM anti-GalNAc-GD1a monoclonal antibody (IgM anti-GalNAc-GD1a mAb) on spontaneous muscle action potentials (SMAP) in a rat spinal cord–muscle co-culture system and the localization of IgM anti-GalNAc- GD1a mAb and calcium channel binding in the rat hemi-diaphragm. Immunohistochemistry of the rat hemidiaphragm showed that IgM anti-GalNAc-GD1a mAb binding overlapped with anti-neurofilament 200 antibody and α- bungarotoxin staining, demonstrating that IgM anti-GalNAc-GD1a mAb was localized at the motor nerve terminal. Moreover, IgM anti-GalNAc-GD1a mAb binding overlapped with anti-Cav2.1 antibody in the nerve terminal. We suggest that the inhibitory effect of IgM anti-GalNAc-GD1a mAb on SMAP is related to the GalNAc-GD1a epitope on P/Q-type calcium channels in motor nerve terminals at NMJs.