Recombinant human granulocyte colony-stimulating factor (G-CSF) was prepared and its granulopoietic effects on mice were examined. When mice were injected intraperitoneally with the G-CSF daily for 14 days, marked granulocytosis occurred in the mice. The number of progenitor cells (CFU-C) was remarkably increased in the spleen. The results suggest that G-CSF plays a central role in granulocyte production in vivo.