The incidence of M-component in 165 patients with leukemia and 64 cases with malignant lymphoma, and the relationship between M-component and the clinical course were studied by a combination of cellulose acetate electrophoretic and agar immunoelectrophoretic analysis. By cellulose acetate electrophoresis, M-component was detected in 3.0% leukemia and in 6.7% of malignant lymphoma. The M-components were classified into two groups according to the electrophoretic pattern. Pattern I showed a clear dense narrow band. Pattern Ⅱ was a faint obscure abnormal band often with two or more associated abnormal bands. Pattern I was found in 6 cases. Except for one case with acute lymphocytic leukemia in which the M-component was accompanied by severe liver disease prier to death, M-components of this type were detected in the sample examined initially and correlated well with the course of the malignant disease. Pattern Ⅱ was found with regression of the tumor in 3 of 4 cases. This type of M-component was suggestive of imbalance in the clonal distribution of immunoglobulin-producing cells as a result of tumor cell infiltration of the immunoglobulin producing system; or of enhancement accompanied by increase of immunoglobulin-producing cells during remission of the tumor. However, in one case of reticulum cell sarcoma, chemotherapeutic factors seems to have contributed to the occurrence of pattern Ⅱ. By agar immunoelectro-phoresis, changes in the IgM precipitation line suggestive of low amounts of M-component and undetectable by cellulose acetate electrophoresis ("abnormal IgM") were observed in 11 cases with leukemia and 6 cases with malignant lymphoma. It seems that these M-components belong to the pattern Ⅱ found with cellulose acetate electrophoresis. In these cases normal parts of the IgM precipitation line were well preserved and two or more abnormalities were frequently found. In leukemia, except for a few cases, this type of M-component appeared transiently during remission of malignant diseases and was associated with severe chronic infectious diseases. In malignant lymphoma, the changes were detected in the first sample without significant complications and lasted for a long time. It seems likely that many factors such as infection, remission of tumor, chemotherapeutic factors and tumor cell infiltration into immunoglobulin producing system are required for the production of the "normal IgM" in leukemia. This is contrary to malignant lymphoma, in which imbalance in the clonal distribution of immunoglobulin-producing cells, detectable as "abnormal IgM", occurs as a result of tumor cell infiltration of the immunoglobulin-producing system. Clinically, the cases with M-component pattern I had short survivals. The survival of patients with pattern Ⅱ varied. In many cases, leukemic potients with "abnormal IgM" survived for a long time. In malignant lymphoma, the survivals were not affected by the presence of M-component of this type. In general, the presence of M-component showed the failure of the immunologic response. However, in some cases such as patients with "abnormal IgM" which appeared at remission of leukemia, occurrence of M-component indicated the process of recovery from the immunodeficient state. To discuss the clinical significance of M-components, the patterns of M-components and basic factors, which arise from imbalance in the distribution of immunoglobulin-producing cells, should be considered.