Hematopoietic growth factors, or hemopoietins, are peptide hormones produced by peripheral blood cells, bone marrow stroma, as well as several other cell types, such as endothelial cells and fibroblasts. Hemopoietins play a central role in regulating peripheral blood cell number and function. By recombinant DNA technology, several of these factors, also called colony-stimulating factors (CSF) or interleukins, are now available in such quantities to allow clinical evaluation. First experiences in clinical trials show that these peptides are capable of stimulating production of various cell types of the peripheral blood by stimulation of bone marrow progenitor cells without significant toxicity for the patient under treatment. Treatment of renal anemia by erythropoietin and reduction of chemotherapy-induced myelosuppression by G- and GM-CSF are already defined indications for these hemopoietins. Further studies are under way to test the indication of these factors in other clinical situations as well as studies investigating the biology, pharmacology, and clinical efficacy of interleukin 3 (IL-3).