With the purpose to see the influences of ACTH and cortisone on the erythropoiesis of bone marrow the author first added ACTH and cortisone directly to the fluid medium of the bone-marrow tissue ctlture of normal rabbits, and then performed the bone-marrow tissue culture in fluid medium of the rabbits previously injected with ACTH or cortisone; and obtained the following results. 1. In the case loaded with ACTH, there is no significant difference from that of the control irrespective of the concentration at 5γ/cc, 100γ/cc, or 500γ/cc, indicating not any influence on the erythropoiesis of the bone marrow. 2. When cortisone is added at the concentration of 5γ/cc, 100γ/cc, or 500γ/cc, at the concentration of 5γ/cc erythropoiesis is slightly accelerated, and at 100γ/cc it is markedly accelerated, while at 500γ/cc the erythropoiesis is rather depressed. 3. In the case of cortisone injection at the dose of 1 mg/kg or 5 mg/kg successively for seven days the erythropoiesis of bone marrow is accelerated as in the case of its direct addition to the bone marrow culture, but the degree of the acceleration is higher than the loading case, especially marked in the injection of 5 mg/kg, but in the injection of 10 mg/kg cortisone the erythropoiesis is on the contrary markedly decreased. 4. In the case of injection of ACTH at the dose of 2mg/kg successively for seven days, the erythropoiesis of bone marrow is accelerated while in the injection of 10 mg/kg successively seven times it is on the contrary markedly reduced. 5. From these it is obvious that the injection of ACTH or cortisone in an appropriate amount accelerates erythropoiesis of bone marrow, while the injection in an excessive amount reduces the erythrocpoietic function. However, as it has been found that ACTH does not act directly upon the erythropoiesis of bone marrow, it is interpreted that in the case of ACTH injection the acceleration of erythropoiesis is due to the action of cortisone secreted secondarily.