Interleukin 3 (IL-3) induces proliferation and differentiation of mast cell progenitors in vitro, whereas it induces granulocytosis in vivo. In this paper, a positive feedback mechanism of granulopoiesis was studied in order to elucidate the granulocytosis induced by IL-3 in mouse. IL-3 induced expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) in total bone marrow cells and a marrow adherent cell population. In fractionated marrow cell populations, a different expression pattern of induction by IL-3 stimulation was observed; GM-CSF was expressed in macrophages and fraction 1 (P<1.061) and 2(1.061<P<1.074) of bone marrow cells fractionated by equilibrium density centrifugation, G-CSF was expressed in macrophages and fraction 2 and 3 (1.074<P<1.097), and interleukin 6 (IL-6) in macrophages and fraction 1 to 3. These results indicate a hierarchical regulation of cytokine production and the existence of a positive feedback mechanism in granulopoiesis. IL-6, induced by IL-3, stimulates stem cells into cycle and induces stem cells to respond to IL-3. The stem cells differentiate to granulocyte-macrophage colony-forming cells by the combined effect of IL-3 and IL-6. IL-3 also induces GM- and G-CSF expression which in turn makes granulocyte-macrophage colony-forming cells differentiate to granulocytes. These factors organize a cytokine network in granulopoiesis.