We investigated, by indirect immunofluorescence, the binding of monoclonal IgM to human peripheral nerve in 25 patients with Waldenstrom's macroglobulinemia and peripheral neuropathy. In 10 cases (40%), an antibody activity against the myelin sheaths was demonstrated. The reactivity was mediated by the F(ab')2 fragments of the IgM. Prior delipidation of nervous tissue was needed to allow full expression of the target antigen(s). In nine cases, the IgM reacted with both peripheral and central myelin of primates, but not mouse or rabbit nervous tissue. In one case, the IgM reacted only with human peripheral nerve. The patients, whose IgM had an antibody activity to myelin antigen(s), had some distinct hematologic and neurologic features. The peripheral neuropathy always antedated the hematologic symptoms by several years. The serum level of the monoclonal IgM was low in all cases, and overt lymphoid malignancy was frequently absent. In other patients with neuropathy, the monoclonal IgM, which lacked antimyelin antibody activity, displayed either cross-idiotypic antigenic determinants or anti-intermediate filament antibody activity. These results taken together emphasize the heterogeneity of antibody activity of the monoclonal IgM from patients with Waldenstrom's macroglobulinemia and peripheral neuropathy.