When granulocyte colony-stimulating factor (G-CSF), purified to homogeneity from mouse lung-conditioned medium, was added to agar cultures of mouse bone marrow cells, it stimulated the formation of small numbers of granulocytic colonies. At high concentrations of G-CSF, a small proportion of macrophage and granulocyte-macrophage colonies also developed. G-CSF stimulated colony formation by highly enriched progenitor cell populations obtained by fractionation of mouse fetal liver cells using a fluorescence-activated cell sorter, indicating that G-CSF probably acts directly on target progenitor cells. Granulocytic colonies stimulated by G-CSF were small and uniform in size, and at 7 days of culture were composed of highly differentiated cells. Studies using clonal transfer and the delayed addition of other regulators showed that G-CSF could directly stimulate the initial proliferation of a large proportion of the granulocyte-macrophage progenitors in adult marrow and also the survival and/or proliferation of some multipotential, erythroid, and eosinophil progenitors in fetal liver. However, G-CSF was unable to sustain continued proliferation of these cells to result in colony formation. When G-CSF was mixed with purified granulocyte-macrophage colony-stimulating factor (GM-CSF) or macrophage colony-stimulating factor (M-CSF), the combination stimulated the formation by adult marrow cells of more granulocyte-macrophage colonies than either stimulus alone and an overall size increase in all colonies. G-CSF behaves as a predominantly granulopoietic stimulating factor but has some capacity to stimulate the initial proliferation of the same wide range of progenitor cells as that stimulated by GM-CSF.