The response of granulopoietic activity in bone marrow depression to a reduction in the demand for erythropoiesis has been examined by means of hypertransfusing C-57B mice which had been exposed to sublethal whole body irradiation. Multiple intraperitoneal injections of 0.5 ml packed red cells from irradiated donors were employed to maintain the haematocrit sufficiently above the normal range to produce significant suppression of erythropoietic activity for the duration of the bone marrow depression. This was associated with elevation above control values of 32--102% in the blood granulocyte count, 22--78% in total cells of the granulocytic series per femur, and up to 44% in total agar colony forming units per femur. Restoration of essentially normal values occurred 13 d after irradiation in contrast to 17--18 d in controls. Single transfusions which produced less suppression of erythroblast numbers per femur resulted in an intermediate degree of improvement in these parameters. Such changes in the granulocyte compartment indicate improved granulopoietic capacity in the hypertransfused group. It is suggested that this effect reflects increased production of granulocyte progenitors due to the reduction in competing demands on the compromised multipotential stem cell compartment for progenitors of the erythroid series. The findings raise the possibility that hypertransfusion might be capable of producing a beneficial effect on granulopoiesis in human bone marrow depression.