Molecular cloning of the genes for several members of a family of glycoproteins, the hematopoietic growth factors (HGFs), has transformed experimental hematology. Molecular probes have been used to investigate the cellular source of the HGFs and the mechanism by which their genes are regulated. Recombinant purified HGFs have been used to investigate their functions in vitro, the binding characteristics of their receptors, and the isolation of certain receptor cDNAs. This increase in knowledge of the cellular biology of the HGFs has led to clinical trials of their efficacy. Specific HGFs or HGF combinations are likely to find a place in the management of several different bone marrow failure states.