Proliferation of acute myeloblastic leukemia (AML) cells in vitro is limited in most cases to a small subset of blasts that have several properties of stem cells. These leukemic colony-forming cells (AML-CFU) generally require addition of exogenous growth factors for proliferation in agar or methylcellulose. These factors can be supplied by media conditioned by phytohemagglutinin-stimulated normal leukocytes or by CSF-secreting tumor cell lines. However, the exact factor or factors required for stimulation of AML-CFU growth have not been defined. We compared the AML-CFU stimulatory activity of a human recombinant GM-CSF with that of GCT-CM, Mo-CM, and the PHA-leukocyte feeder system in 15 cases of AML. In each of the 12 cases that required exogenous growth factors for maximum AML-CFU growth, recombinant GM-CSF could replace either GM-CSF or Mo-CM, and could partially replace the PHA-leukocyte feeder system. These results indicate that this GM-CSF is a growth promoter of AML-CFU in these culture systems.