With the use of the simple bone-marrow tissue culture, the method devised in our laboratory, in the medium composed of serum and vitamin B(12), the author studied the neutral red vital staining of neutrophils in the bone marrow of both normal persons and patients with various blood diseases. These results were compared with those obtained by the conventional method using the medium composed of plasma mixed with heparin and chick embryo juice; and the author arrived at the following conclusions: 1. The average stain of mature neutrophils in the case of normal persons reaches the maximum of 1.12 within 4 hours after the addition of neutral red solution, fading gradually thereafter. 2. In the case of leukemia mature neutrophils in the bone marrow are stained more deeply at an early stage as compared with those of normal persons; and in the acute leukemia they fade at an early stage, and in the chronic leukemia they fade more slowly than those of acute leukemia. 3. In the case of hypoplastic anemia mature neutrophils in the bone marrow are stained deeper and earlier than those is normal persons, and also they fade more quickly. 4. In the case of agranulocytosis mature neutrophils are stained more lightly than those in normal persons. 5. In the case of essential hypochromic anemia mature neutrophils in the bone marrow are stained more deeply and earlier than those in normal persons, but fading more slowly. In the case of pernicious anemia these neutrophils are stained more deeply both before and after the treatment than those in normal persons, but as for the fading before and after treatment, the former is slightly faster than the latter. 6. These results indicate that, although this method is slightly inferior to the conventional method from the standpoint of neutral red vital staining, it is rather a superior method for clinical application because of its simplicity in manipulation the same as in the study of the carbon-particle phagocytosis.