Abstract
Estradiol valerate antagonizes the stimulating effect of erythropoietin on the plasma clearance of Fe59, the incorporation of Fe59 into erythrocytes and the increase of red cell volume. 17[alpaha]-Hydroxyprogesterone caproate and testosterone propionate were able to antagonize the inhibition of erythropoietin by estradiol in polycythemic mice. The mechanism of action appeared to us to be primarily at the stem cell level.