Recombinant human granulocyte colony-stimulating factor (G-CSF) is a nonglycosylated protein produced in Escherichia coli using recombinant DNA technology. G-CSF was first defined in vitro as a relatively selective stimulator of pure granulocyte colonies from normal marrow and as a factor that induces differentiation of leukemic cell lines. Additional studies have shown that it has significant effects on primitive marrow stem cells as well as on the differentiated cells of the granulocyte-macrophage pathway enhancing phagocytosis, superoxide release, antibody-dependent cellular cytotoxicity, and migration of both neutrophils and monocytes. The most extensively studied clinical application of G-CSF has been in chemotherapy-induced myelosuppression, where it was shown to reduce the duration of severe neutropenia, the incidence of febrile neutropenic episodes, the overall duration of intravenous antibiotic therapy, and the length of hospitalization. G-CSF has also been shown to correct primary and acquired forms of neutropenia, to accelerate neutrophil recovery after bone marrow transplantation, and to mobilize stem cells in peripheral blood or hemopoietic rescue. G-CSF is well tolerated, mild to moderate bone pain being the most frequently reported adverse side effect. The clinical applications of G-CSF are likely to expand as more information emerges from continuing clinical trials.