Daily administration of purified human granulocyte colony-stimulating factor produced by E. coli (Hu-G-CSF) accelerated the recovery from neutropenia induced by total-body irradiation in mice. Granulocyte-macrophage progenitors assayed as colony-forming unit granulocyte/macrophage (CFU-GM) in spleen were markedly increased in number by Hu-G-CSF, and the accelerated recovery from neutropenia may be due to the stimulation of granulocytopoiesis by the G-CSF from progenitor cells.