The colony-stimulating factors (CSFs) principally involved in the production of neutrophils and monocytes are granulocyte CSF, granulocyte-macrophage CSF, macrophage CSF, and interleukin 3 (sometimes called multi-CSF). The natural response to inflammation and infection in the immunocompetent host probably involves all of these CSFs. CSFs can be used as pharmacological agents to accelerate the production of neutrophils and monocytes/macrophages and to enhance mechanisms of host defense. Rapidly accumulating evidence appears to justify the use of CSFs for the prevention of fever and infections in several clinical settings, such as chemotherapy-associated neutropenia, bone marrow transplantation, and severe chronic neutropenia. Trials of CSF treatment of infections in settings not including neutropenia are under way.