For the purpose of revealing whether the sensitivity of the erythropoiesis to actinomycin D (AMD) differs among different animal species, and to see the acting site of AMD on erythroid cell specialization stage, the author observed the hourly change of the blood cell counts and bone marrow cells after AMD administration to mice, rats and rabbits, and obtained the following results: 1. The data indicated that the erythropoiesis of ra bbit is sensitive to AMD, as much as that of mice, while the rat is resistant to AMD, and its erythropoiesis is not affected by the similar dose of AMD as in the case of mouse and rabbit. 2. The morphologic observations on the eradication process of erythroblasts in the bone marrow of mice and rabbits indicates that AMD acts as to inhibit the transformation of the stem cell to the proerythroblast but not on the erythroblast in the course of specialization. The time required for the eradication coincided with the time of the proerythroblast to the mature red cell. 3. Discussion has been made on the possibility of the common stem cell to erythroid and granulocytic cells in relation to the lymphoid cells in bone marrow and their blast form.