Hematopoietic growth factors have transformed the practice of oncology. The two major factors in clinical use are recombinant human (rh) granulocyte colony-stimulating factor (G-CSF, filgrastin [Neupogen]) and granulocyte-macrophage colony-stimulating factor (GM-CSF, sargramostim [Leukine]). These factors differ significantly in their role in hematopoiesis and the regulation of mature effector cell function. G-CSF regulates both basal and neutrophil production and increased production and release of neutrophils from the marrow in response to infection. GM-CSF mediates its effects on the neutrophil lineage through its effects on phagocytic accessory cells and its synergy with G-CSF, but it does not appear to have a role in basal hematopoiesis. Part 1 of this two-part series focuses on the use of the myeloid growth factors rhG-CSF and rhGM-CSF to shorten the duration of chemotherapy-induced neutropenia and thus prevent infection in cancer patients. In randomized trials, rhG-CSF has consistently decreased the duration of neutropenia during all cycles of chemotherapy and reduced the risk of infection by 50% or more. Trials of rhGM-CSF have not reported consistent results.