By means of the simple bone-marrow tissue culture in the medium composed of serum and vitamin B(12), the method of our own device, the author studied the carbon-particle phagocytosis of neutrophils both in the bone marrow of normal persons and patients with various blood diseases, and compared these results with those obtained in the tissue culture using the conventional medium composed of plasma mixed with heparin and chick embryo juice. The results are as follows: 1. The average phagocytic capacity of mature neutrophils of healthy persons reaches the maximum of 1.03 after 6 hours, culture, and it slowly falls thereafter. 2. In the case of leukemia the phagocytic capacity of mature neutrophils in the bone marrow is lower than that in the case of normal persons, but when the same in acute leukemia is compared with that in chronic leukemia it is far lower in the former than in the latter. 3. The phagocytic capacity of mature neutrophils is greatly decreased in the case of hypoplastic anemia, and moderatery in agranulocytosis and Banti's disease. Moreover, in the case of Banti, s disease, when the serum of either normal persons or patients with this disease is used in the medium, the phagocytic capacity falls in either case, but the serum of normal persons tends to prove somewhat to reinstate the phagocytic power of mature neutrophils. 4. In the cases of essential hypochromic anemia and pernicious anema the phagocytic capacity of mature neutrophils decreases to an intermediate degree in both. 5. By our simple bone-marrow tissue culture method it is possible to observe the cells in bone marrow for quite long period of time. Moreover, when compared with the conventional method of the tissue culture, this is only slightly inferior, but from its very simplicity of manipulation, it is really more useful for the clinical application than the conventional method.