The author describes the results obtained in the series of tissue culture in fluid medium using the bone marrow from normal persons and patients with various blood diseases as materials supplemented with iron or vitamin B(12), in ordar to observe the alteration in the number of sideroblasts, and discusses about the iron metabolism of erythroblasts. 1 In the bone-marrow tissue culture of normal persons the iron addition to the culture slightly increases the number of sideroblasts. 2. In the cases of bone-marrow tissue culture of idiopathic hypochromic anemia, hook worm anemia, Banti's disease and malignant tumor, sidereblasts are increased considerably in proportion to the degree of iron deficiency state in these diseases by the iron addition. 3. In the cases of hypoplastic anemia and liver cirrhosis, sideroblasts are hardly or only slighly increased. 4. In the cases of leukemias the incresse in the number of sideroblasts in vitro by the addition of iron differs widely from a considerable increase to a very small incease. 5. Despite the fact that polycythemia vera shows a low rate of sideroblast appearance, there can berecognized no appreciable increase in the number of sideroblasts by the addition of iron. 6. In the cases of iron deficiency and the lowered or accelerated functions of bone-marrow, each of them reveals certain different patterns of alteration in the number of sideroblasts and to this phenomenon stainable iron of erythroblasts before addition plays an important role. 7. However, in the cases of the bone-marrow tissue culture supplemented with vitamin B(12) sideroblasts are hardly affected, and therefore, vitamin B(12) has almost no effect on the iron metabolism of erythroblasts.