Recombinant rat stem cell factor (rrSCF) and recombinant human granulocyte colony-stimulating factor (G-CSF) coinjected for 1 week in rats cause a synergistic increase in mature marrow neutrophils accompanied by a striking decrease in erythroid and lymphoid marrow elements. The spleens of the same rats show increased granulopoiesis as well as increased erythropoiesis as compared with the spleens of rats treated with either growth factor alone. Splenic extramedullary erythropoiesis may act to compensate for the decrease in marrow erythropoiesis. The coinjection of rrSCF and G-CSF causes an increase in marrow mast cells at the end of 1 week, but the increase is much less than in rrSCF-alone-treated rats. The combination of rrSCF and G- CSF increases the rate of release of marrow neutrophils into the circulation and causes a dramatic synergistic peripheral neutrophilia, beginning especially after 4 days of treatment. Colony-forming assays of all experimental groups showed a synergistic increase in colony- forming unit granulocyte-macrophage (CFU-GM) in the marrow, but not in peripheral blood, after coincubation with SCF plus granulocyte- macrophage CSF (GM-CSF) as opposed to GM-CSF alone, showing anatomic compartmentalization between a more primitive marrow CFU-GM subset and a more mature peripheral blood CFU-GM subset. In vivo daily administration of SCF plus GM-CSF results in a synergistic increase in marrow neutrophils, but not the striking synergistic increase in circulating neutrophils that is observed with SCF plus G-CSF.