Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) enhance the antimicrobial functions of mature neutrophils. G-CSF differs from GM-CSF in its specificity of action on developing and mature neutrophils, its effects on neutrophil kinetics, and its toxicity profile. The toxicity profile of recombinant (r) GM-CSF is consistent with priming of macrophages for increased formation and release of inflammatory cytokines, whereas rG-CSF induces production of antiinflammatory factors, such as interleukin-1 receptor antagonist and soluble tumor necrosis factor receptors, and is protective against endotoxin- and sepsis-induced organ injury. The low toxicity of rG-CSF, results of animal models of infection, and extensive experience with neutropenic subjects have promoted clinical studies in nonneutropenic subjects, which indicate that rG-CSF may be beneficial as adjunctive therapy for treatment of serious bacterial and opportunistic fungal infections in nonneutropenic patients, including those with alterations in neutrophil function.