Numerous studies have shown that interferon-gamma (IFN gamma) inhibits the proliferative effects of colony-stimulating factors (CSFs) on human bone marrow cells. In the present study we investigated the effects of IFN gamma and other described inhibitory factors on the proliferation of highly purified CD34+ human hematopoietic progenitor cells (HPC) in response to recombinant CSFs. While transforming growth factor-beta (TGF beta) and IFN alpha were highly inhibitory, IFN gamma strongly potentiated interleukin-3 (IL-3) and, to a lesser extent, granulocyte-macrophage-CSF (GM-CSF) induced growth of CD34+ HPC. IFN gamma had no significant proliferative effect per se, and did not affect granulocyte-CSF (G-CSF)-dependent cell proliferation. Within 10 days the number of viable cells generated in the presence of IL-3 + IFN gamma was two times higher than in the presence of IL-3 alone. Limiting dilution analysis showed that IFN gamma acts directly on its target cell to increase the frequency of IL-3-responding cells without affecting the average size of the IL-3-dependent clones. Enhanced frequency of IL-3- and GM-CSF-responding cells was also observed in colony assays where the addition of IFN gamma increased by twofold to threefold the number of granulocyte colony-forming units (CFU-G), macrophage CFUs (CFU-M), granulocyte-macrophage CFUs (CFU-GM), and mixed erythroid (E-MIX). In contrast, IFN gamma did not affect the generation of erythroid burst-forming units (BFU-e) in such cultures. In longer-term culture, the combination of IFN gamma and IL-3 did not alter the lineage distribution of the cells when compared with IL-3 alone. However, after 15 days, when mature cells were present in the cultures, IFN gamma displayed cell concentration-related growth-inhibitory effects. Thus, IFN gamma appears to stimulate the early stage of myelopoiesis by enhancing the frequency of growth factor-responding cells but, unlike tumor necrosis factor alpha (TNF alpha), does not alter cell differentiation.