Intravenous (IV) administration of 5-fluorouracil (5-FU) caused severe hematopoietic injury in mice. Daily administration of purified human recombinant granulocyte colony-stimulating factor (rG-CSF) accelerated recovery from neutropenia in these injured mice. Granulocyte-macrophage progenitors (CFU-GM) in spleen were markedly increased by rG-CSF, while rG-CSF was less effective on the regeneration of CFU-GM in femoral marrow. The accelerated recovery from neutropenia in the mice might be mainly due to the stimulation of granulopoiesis in spleen by rG-CSF.