A patient had Waldenström's macroglobulinemia with increased serum levels of heterogeneous or "polyclonal" IgG in addition to elevated homogeneous or "monoclonal" IgM levels. A low dosage of cyclophosphamide was begun and has been continued to the present. Serum IgM levels fell promptly followed by a fall in IgG levels. After both the IgM and IgG levels had returned to normal no further fall occurred. Normal serum IgA levels were unaffected by the treatment. The fall in serum IgM and IgG values was accompanied by a disappearance of lymphocytes from the hepatic portal areas and a slight decrease in the lymphocytic infiltration of the bone marrow. A complete clinical remission began 6 months after therapy was begun and has continued to the present, 3 years later.