The results presented in this paper demonstrate that recombinant human granulocyte-colony stimulating factor (rhG-CSF) is a potent myelopoietic growth and differentiation factor in vivo. RhG-CSF was able to shorten the time period of neutrophil recovery in both cyclophosphamide (CY)-induced myelosuppression and following bone marrow transplantation (BMT) in primates. Its ability to significantly shorten the period of chemotherapy-induced bone marrow hypoplasia may allow clinicians to increase the frequency or dosage of chemotherapeutic agents. In addition, the increase in absolute numbers of functionally active neutrophils may have a profound effect on the rate and severity of neutropenia-related sepsis. Furthermore, the activities reported here indicate a potential role for rhG-CSF in the treatment of patients with myelodysplastic syndrome, congenital agranulocytosis, radiation-induced myelosuppression, and after bone marrow transplantation.