Purified peripheral blood hematopoietic progenitors from children in early remission from cancer respond to recombinant human interleukin-3 (IL-3), but not to granulocyte colony-stimulating factor (G-CSF). With these purified cells as a target, we studied the effect of recombinant human interferon-gamma (IFN-gamma) on progenitor growth, using both liquid-suspension limiting dilution assay (LDA) and regular methylcellulose culture of progenitors. We found that in LDA with IL-3, IFN-gamma directly stimulated the growth of blood progenitors in a dose- dependent manner with single-hit kinetics, whereas IFN-gamma suppressed the growth of G-CSF-supported progenitors obtained from bone marrow. The stimulatory effect was also observed in methylcellulose culture, but the addition of antibodies for G-CSF, granulocyte-macrophage CSF, IL-1 alpha, IL-1 beta, IL-6, or tumor necrosis factor did not result in a decrease of the colony number, supporting further the possible direct effect of IFN-gamma on progenitor growth. These results suggest that the inhibitory effect of IFN-gamma on hematopoietic progenitors is limited to those in an advanced stage of maturation. IFN-gamma may be one of the essential lymphokines upregulating the growth of human hematopoietic progenitor cells.