Injection of foreign serum into mice increases erythrocyte formation as evaluated by ferrokinetics studies. When erythropoiesis is depressed either by fasting or plethora, the wave of erythropoiesis that follows a transient increase of endogenous erythropoietin is clearly enhanced by pretreatment of the recipients with foreign serum. The response includes a restoration of the responsiveness of the spleen of fasted mice to endogenous erythropoietin stimulation. These changes seem related to an effect of foreign serum on the transition of primitive hematopoietic progenitors into erythroid-committed progenitors.