A case of mu-heavy chain disease (HCD) is described. The patient, a 40-year-old man, presented with an intracranial tumor. The bone marrow of this patient showed infiltration with both plasma cells and lymphocytes. The majority of plasma cells were vacuolated and the result of immunoelectrophoresis of serum protein revealed an arc with anti-IgM antiserum and an additional rapid migrating arc of different mobility with anti-kappa antiserum. The urine contained a kappa-type Bence Jones protein. An enzyme-linked antibody study showed that the majority of plasma cells in bone marrow contained both mu and kappa antigenic determinants in their cytoplasm. On Sephadex G-200 gel filtration, the monoclonal IgM-kappa protein and the mu-chain fragment were detected in the serum, suggesting the combined features of mu-HCD and macroglobulinemia. The molecular weight of the mu-chain fragment was approximately 45,000 daltons. The intracranial tumor completely disappeared after irradiation therapy. However, he died 1 year later after development of a huge abdominal tumor.