Colony stimulating factors and interleukins regulate proliferation, differentiation, and functional activation of hematopoietic cells of multiple lineages. These hematopoietic growth factors are proving effective in vivo in stimulation of granulopoiesis in clinical situations associated with myelosuppression. G-CSF and GM-CSF promote accelerated granulocyte recovery following chemotherapy, or allogeneic or autologous bone marrow transplantation, in patients with cancer. In congenital defects of granulocyte production or in acquired disorders such as idiopathic neutropenia or aplastic anemia, CSF administration can lead to recovery of functioning granulocytes. This has resulted in a reduction in the morbidity and mortality associated with these diseases and now permits both a dose and a schedule intensification of chemotherapy. In myeloid leukemia and myelodysplastic syndromes, CSF treatment, particularly G-CSF, has proved effective for certain patients in improving neutrophil, platelet, and occasionally red cell production while reducing blast cells. The recombinant growth factors are generally well tolerated with few limiting toxicities at dose levels that effectively stimulate hematopoiesis.