A number of human hematopoietic growth factors have been genetically cloned and recombinant proteins produced. Several phase I and II clinical trials have already been published and results from phase III studies are becoming available. The use of erythropoietin to alleviate chemotherapy-induced myelosuppression is being tested. Granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor have been extensively studied in patients receiving chemotherapy at standard or escalated doses and after bone marrow transplantation and appear to ameliorate chemotherapy-induced neutropenia and to speed bone marrow engraftment after high-dose cancer therapy. Interleukin-3 and interleukin-6 are quite early in their clinical development, but appear able to alleviate post-chemotherapy thrombocytopenia.