In the study of monocytic leukemia by means of bone marrow tissue culture, the author has obtained the following results: 1. The rate of relative increase of the growth area fell down approximately to one third that of the control, but the cellular density in the growth area increased remarkably; and the boundary of the growth area was sharply defined from the surrounding area, indicating a marked acceleration of the cellular proliferative activity in this disease. 2. From the very characteristics of morphological findings, process of maturation, and superiority in numbers over other cellular series, in monocytic series appearing in the growth area, it is easy to make differential diagnosis of monocytic leukemia from other leukemias. 3. The juvenile type of the cells in monocytic series makes it possible to differentiate from other juvenile calls by the characteristic findings in morphology, thus enabling us to distinguish monocytic leukemia from other leukemias. 4. Monocytic leukemia and reticuloendotheliosis that have been commonly believed for a long time to be the same are really two entirely different diseases.