Granulocyte colony-stimulating factor is a potent stimulator for neutrophils in the circulation as well as those progenitors to be increased and be grown in the bone marrow. Recombinant forms of this colony-stimulating factor, therefore, are widely using in the various kinds of clinical fields such as for the treatment of bone marrow suppression after cancer chemotherapy and for the treatment of myelodysplastic disorders. One of the physiological function of granulocyte colony-stimulating factor is to activate neutrophils enough to fight against invaded microbes and others, thus it is hoped to be used for the treatment of various kinds of infectious diseases even while the host has a normal number of circulating neutrophils and normal bone marrow. The future possible clinical usage of granulocyte colony-stimulating factor as for the immunomodulation against various kinds of insults to the human is reviewed.