A 56-year-old man was admitted to our hospital in November, 1991 because of hyperproteinemia and anemia. Total protein showed 12 g/dl and serum immunoglobulins were as follows; IgG 974 mg/dl, IgA 142 mg/dl, IgM 9270 mg/dl. M-component was identified as IgM-kappa with immunoelectrophoresis and serum viscosity indicated 6.9. Although the patient had no history of severe streptococcal infection, his serum showed very high activity of ASLO (6890 IU/ml). Bence Jones protein was detected in the urine and determined to be of kappa-type. Plasma cells occupied 43% of bone marrow nucleated cells and their cytoplasms were stained with FITC-labeled anti-IgM and anti-kappa antibodies. X-ray examination disclosed punched out lesions in the skull. Consequently, the patient was diagnosed as having IgM-myeloma. At first, VCAP regimens were given after plasmapheresis, but had no effect. Therefore, the patient was treated with CHOP protocol and the serum IgM decreased in amount and the proportion of bone marrow myeloma cells got down to 17%. ASLO titer also decreased in parallel with IgM. These findings suggest that IgM in this case had ASLO activity.