The Colony Stimulating Factors (CSFs) are a family of haemopoietic hormones that likely share a common ancestral origin and stimulate white blood cell development. They display unique but overlapping biological functions and stimulate the survival, proliferation, differentiation and functional activation of granulocytes and monocytes/macrophages and their precursor cells in vitro and in vivo. Each hormone has been purified and produced in active recombinant form. Recombinant G-CSF and GM-CSF are now being used around the world in a variety of clinical situations (e.g., in conjunction with chemotherapy and bone marrow transplantation) to promote the formation and function of these leukocytes. These molecules are among the first of a new generation of biological agents that will impact enormously on clinical medicine.