By performing both bone-marrow tissue culture and peripheral-blood tissue culture by cover-slip method the author made vital observations of leukemic cells and studied the fixed and stained specimens of cultured tissues; and also observed the mode of maturation and increase of leukemic cells by the tissue culture of peripheral leucocytes of leukemic patients in fluid medium; and obtained the following results: 1. In the tissue culture by the cover-slip method, monoblasts, promonocytes and monocytes present peculiar vital findings and movement pattern so that bone-marrow tissue culture of leukemic patients by the cover-slip method has been confirmed to be the most excellent method for an accurate diagnosis of monocytic leukemia. 2. The findings on the growth zone of the cultued tissue in the fixed and stained specimens show the picture specific to monocytio leukemia, and promonocytes and monocytes present the picture of these cells in actual motion. Although the each finding is somewhat inferior to that of smeared-stained specimens, it is, nevertheless a fair picture. 3. Even in the peripheal-leucocyte tissue culture by the cover-slip-method in monocytic leukemic patients, it gives a picture almost identical with that of the bone-marrow tissue culture. This method can take place of the bone-marrow tissue culture quite well even in the case where the bone-marrow tissue culture is not feasible, and for the diagnosis of this disease it is just as useful as the bone marrow culture. 4. For the peripheral-leucocyte tissue culture of monocytic leukemia in the fluid medium, the author used the serum of healthy person having the same type blood with that of the patient according to a modified method of Osgood's devised by the author. The increase in the leucocyte count by the tissue culture in fluid medium reaches its maximum between 5 and 6hours after the culture, and the mitotic picture can be observed most numerously during this period. The decrease in monoblasts and increase in monocytes and promonocytes can be observed along with the lapse of the culture time. Namely, the mitosis and maturation of leukemic cells have been found to be taking place in the medium. 5. As the matured type of leukemic cells increases in number and the stained-specimen findings are good in the tissue culture of fluid medium, this is a method for even those who are not so skilled in vital observations for the differential diagnosis of leukemia. In addition this method will be quite valuable as a supplementary method for the cover-slip method for the diagnosis of blastic leukemia in which ther appear extremely a small number of the matured type.