In B6D2F1 female mice a single dose of 10 µg. of actinomycin will suppress normal erythropoiesis. In polycythemic mice 2 µg. is enough to prevent the stimulatory effect of 0.25 A units of erythropoietin. The curves of suppression and recovery versus time support the hypothesis that erythropoietin acts for a short time in an early stage of erythropoiesis; after this early stage developing cells are no longer so sensitive to actinomycin. Recovery, even from repeated doses, is rapid and adequate. Amounts of actinomycin which are adequate to abolish erythropoiesis do not prevent the new appearance of erythropoietin in the plasma of hypoxic mice.