In some patients with plasma cell dyscrasia and neuropathy, there are IgM M proteins that react with the myelin-associated glycoprotein (MAG). We used an enzyme-linked immunosorbent assay (ELISA) system to detect anti-MAG IgM antibodies. Reactivity with human MAG by ELISA correlated with demonstration of anti-MAG IgM antibodies by the "immunoblot" technique. Human MAG was more effective than bovine MAG as antigen, and there was no significant reactivity with mouse MAG. The ELISA system is a simple and convenient method for detecting anti-MAG IgM antibodies.