The experiments were carried out on animals in which erythropoiesis was stimulated. The rats were anaemized by blood letting or they received subcutaneous injections of erythropoietic filtrate of anaemized sheep plasma. The investigations demonstrated that enhanced 54Mn incorporation into the erytrocytes and bone marrow is induced by stimulation of the erythropoiesis process, whereas the raised 54Mn level in the plasma of anaemized rats is associated with blood loss and not with erythropoiesis.