The author studied the effects of various vitamins on the cells of the erythrocyte series by performing the bone-marrow tissue culture of normal rabbits, and obtained the following results. 1. It has been found that vitamin B(1), cocarboxylase, vitamins B(2) and C, nicotinic acid, and vitamin K possess no power directly increasing the number of the erythrocyte series in the bone marrow. 2. Vitamin B(2) seems to be activated in vivo and transformed to FMN (flavin mononucleotide) or FAD while nicotinic acid in the form of DPN (diphosphophopyridine nucleotide) or TPN (triphosphopyrine nucleotide) can only play a role in the hematopoiesis, and it is believed that these vitamins at least in their original forms can not act directly on the bone marrow as to accelerate erythropoiesis. 3. Folic acid is first activated in vivo and trasformed to citovorum factor, and it is this latter factor that acts on the bone marrow as to accelerate the maturation of erythrocytes, but it does not have any effect on the erythropoiesis of the bone marrow in its original form. 4. FAD and vitamin B(6) have been found to have the power to accelerate erythropoiesis of the bone marrow though only slightly. 5. It has been recognized that vitamin B(12) acts directly on the bone marrow and markedly accelerates erythropoiesis of the bone marrow. 6. All these vitamins tested have no accelerating offect on the hemoglobin synthesis.