TNF-alpha has been shown to antagonize the proliferative effects of growth factors present in crude conditioned media from PHA-stimulated leukocytes or cell lines on the clonogenic cells of acute myeloblastic leukemia (AML) (19,21). In the present study, we investigated the responses of AML blasts to TNF-alpha in the presence of defined growth factors (recombinant granulocyte/macrophage-CSF [rGM-CSF], recombinant granulocyte-CSF [rG-CSF], rIL-3, and rIL-1) and under conditions described for autocrine stimulation (32). While TNF-alpha antagonized the stimulatory effects of G-CSF and IL-3 on blast progenitors, TNF-alpha did not affect blast colony formation in the presence of IL-1. Unexpectedly, TNF-alpha significantly enhanced blast proliferation in the presence of GM-CSF. Further, TNF-alpha also acted synergistically with an endogenous source of growth stimulatory signal to promote proliferation of blast clonogenic cells. Thus, on human leukemic cells, TNF-alpha appears to be a molecule that is at least bifunctional, having the ability to either support or inhibit cell proliferation, depending on the other growth factors present. It is postulated that the proliferative response of blast progenitors to TNF-alpha under conditions that favor autocrine stimulation may represent one property that allows the cells to escape from negative regulation and proliferate in AML.