The influences of 1) the sera of the patients with various blood diseases, 2) aminoacids, and 3) vitamins, on the rabbit bone marrow culture in fluid were observed. Ⅰ. The influences of the serum a) By addition of any sorts of serum, there were no increase of nueleated cell. This fact might be caused, i) in general, by the ripening of a great number of erythroblasts into normal erythrocytes, and ⅱ) in some blood diseases, by the depressive action of some factor contained in the serum. b) Erythrocytes increased in number, especially by the exist of the serum of healthy individual, but the serum of the patient with hypoplastic anemia or Banti’s syndrome arrested the increasing of erythroeytes. c) Without serum hemoglobin could not be synthesised, but the existence of the serum, even the serum from hypoplastic anemia, caused the hemoglobinsynthesis. This means that iron and the some factor contained in the serum play an important roll in the hemoglobinsynthesis. d) As for the attitude of reticulocytes to the influences of the serum, there were no differences between the serum of healthy individual and of the patients with blood disease. From these findings, the fact that the sera of the patients with hypoplastic anemia, Banti’s syndrome or Werlhof’s purpura contain the depressive factor to the bone marrow function is doubtless. Monocytic leukemia also contains the depressive factor, but this factor depressed chiefly erythropoiesis. The depressive function of the sera of the patients with anemia caused by anchylostomiasis, essential hypochromic anemia or chronic myelogenous leukemia, are slight, and therefore as the cause of anemia by anchylostomiasis "arresting of blood in the bone marrow sinusoid", observed by Dr. Hiraki et al., may be more important.