There are at least four known human colony-stimulating factors (CSFs) which can stimulate proliferation of specific types of hematopoietic progenitor cells in vitro and in vivo: IL-3, GM-CSF, G-CSF, and M-CSF. In animal studies and preliminary human trials, at least two of these CSFs have been shown to accelerate granulocyte recovery following cytotoxic chemotherapy. The CSFs may ultimately have far-reaching effects on the practice of oncology, permitting higher doses of cytotoxic drugs and radiation, allowing design of more aggressive drug schedules, decreasing infectious complications and hospital stays, and diminishing the morbidity of marrow transplantation. The author describes the functions, cellular sources, and potential uses of the colony-stimulating factors.