By performing the peripheral-leucocyte culture of patients with various leukemias, leukemoid reaction, and hypoplastic anemia, the author observed the tissue growth and the function of neutrophils in the culture; and obtained the following results: 1. In the case of chronic myelogenous leukemia, the tissue growth is composed of double layers of growth zones, namely, one, the inner young-cell zone with a clear-cut boundary and high cell density, and the other, the outer mature-cell zone with less cell density; but the young-cell zone gradually grows smaller with the lapse of time as these young-cells mature. The function of neutrophils is slightly inferior to that in the case of normal persons. 2. In the cases of the acute myelogenous and the lymphocytic leukemias as well as monocytic leukemia, the tissue growth area is sharply demarcated, forming no outer maturecell zone. As for the cell density, it is highest in the case of lymphocytics, and in the case of monocytic it is relatively low; and with the lapse of culture time immature blasts are seem maturing, each type of cells presenting their peculiar movement pattern and thus it is possible to distinguish different types of cells. The function of neutrophils in acute form is extremely lower than that of chronic form. 3. In the slice specimens prepared during the peripheral-leucocyte culture of leukemia many mitotic pictures were recognized. 4. In the case of leukemoid reaction the pattern of tissue growth and the function of neutrophils are identical with those in the case of normal persons, making it easy to differentiate this disease from leukemia. 5. In the case of hypoplastic anemia, the tissue growth area presents a coroma-like pattern of growth the same as in the case of normal persons, but the growth area is smaller and the cell density is less, and moreover, the function of neutrophils is markedly diminished. From these results it has been proven that clinically the peripheral-leucocyte culture is sufficiently worthy of use for the differential diagnosis of various blood diseases.