In our observation of leukemia and malignant lymphoma, many cases with slight changes in the IgM precipitation line ("abnormal IgM"), suggestive of small amounts of M-component, were detected by immunoelectrophoretic analysis. In these cases, determination of the light chain type seemed necessary for the diagnosis of M-component. However, typing was difficult by conventional methods because of the low amount of M-component. In this report, a new method for typing of the light chain of the IgM type of M-component, using FITC labelled anti-light chain specific antisera, is discussed. With this method, the light chain type of M-component was determined by the difference in strength of FITC specific fluorescence between the reaction of FITC conjugated anti-kappa and -lambda chain sera at the abnormal parts of the immune precipitation, which were prepared by agar immunoelectrophoresis using antiheavy chain specific antisera. This method was applied to 11 cases with usual M-component, in which the light chain types were determined only by agar immunoelectrophoresis utilizing anti-light chain specific antisera or by combination with gel-filtration. In all cases, the types were determined in agreement with previous methods. Compared with cellulose acetate electrophoresis or agar immunoelectrophoresis using anti-heavy chain specific antisera, the immunofluorescent method had a sensitivity similar to agar immunoelectrophoresis and more sensitive than cellulose acetate electrophoresis. This method was applied to 7 cases with "abnormal IgM" which were found in leukemia and malignant lymphoma. In all 7 cases, differences in the strength of FITC specific fluorescence at the abnormal parts of the precipitation line were observed. As a result, 1 case with both kappa and lambda types, 2 cases with the kappa type and 4 cases with the lambda type of M-component were found. These results indicate the usefulness of this method for light chain typing of low amounts of M-component with IgM type. It seems likely that the slight changes in the IgM precipitation line, detected in leukemia and malignant lymphoma, result from M-components.