Human macrophage colony-stimulating factor (hM-CSF) has been isolated from normal human urine and purified to a homogeneous protein. The effect of hM-CSF on granulopoiesis was investigated in BALB/c mice transplanted with a suboptimal number of bone marrow cells. Lethally irradiated (7.8 Gy) mice were transplanted with 1 x 10(6) syngeneic mouse bone marrow cells and treated with a daily intraperitoneal dose of 64 micrograms/kg of hM-CSF for 5 days following the transplant. The hM-CSF injection resulted in stimulation of the recovery of blood neutrophils as well as an increase in the number of granulocyte-macrophage progenitor cells (CFU-GM) in the femur and spleen. The survival of lethally irradiated mice was dependent on the cell number transplanted; most mice transplanted with 2 x 10(4) cells died within 2 weeks. The recovery of hematopoiesis in mice transplanted with 2 x 10(4) cells was modestly but significantly stimulated by hM-CSF administration initiated from 5 days before or 1 day after transplantation for a 5-day period. Furthermore, the hM-CSF administrations markedly reduced the mortality in these mice during the early period after the transplantation. Since anaerobic bacteria were frequently detected in arterial blood immediately before the deaths but were not found in the surviving mice, it is speculated that early deaths occurring within 2 weeks after the transplant may be caused by opportunistic infections, and hM-CSF injection may prevent these mortal infections through its stimulating effect on monocyte-macrophage functions that are responsible for the production of hematopoietic regulators.