Quantitative, immunopathological, light and electron microscopic studies of superficial peroneal nerve biopsies from 31 patients with IgM monoclonal gammopathy were carried out. Six patients had Waldenström's macroglobulinemia and 25 had IgM monoclonal gammopathy of undetermined significance. Serum samples from 28 of these patients were assayed for anti-myelin-associated glycoprotein (anti-MAG) activity. Anti-MAG activity was found in 25 of the samples. There was a relationship between the widening of some myelin lamellae observed on ultrastructural examination and the serum anti-MAG activity (23 cases). Immunopathological examination showed IgM binding to myelin sheaths in 17 cases.