Recombinant human granulocyte colony-stimulating factor (rhG-CSF) was injected sc, twice daily, into normal adult male BDF1 mice for a period of 4-5 days in doses ranging from 10 to 2500 micrograms/kg/day. The number of peripheral blood cells and their morphology was examined daily from the beginning of the experiment for 6 days. On the 5th day of treatment parameters such as spleen weight and cellularity, bone marrow morphology and cellularity, content of granulocyte-macrophage colony-forming cells (GM-CFC) or multipotential stem cells (CFU-S) in spleen and marrow, and also histology of lung and liver were examined. We observed a dose-dependent increase in the number of blood neutrophils, and an increase in weight, cellularity, and numbers of GM-CFC and CFU-S in the spleen. In the bone marrow, cellularity decreased to 40% of normal. Numbers of GM-CFC and CFU-S in marrow were also decreased, and examination of marrow morphology revealed an inhibition of erythropoiesis.