Previous studies have shown that tumor necrosis factors (TNFs) inhibit the proliferative effects of crude or purified colony-stimulating factors (CSFs) on low density human bone marrow cell fractions. In the present study we investigated the effects of TNFα on the growth of highly purified CD34 human hematopoietic progenitor cells (HPC) in response to recombinant CSFs. In short-term liquid cultures (5 to 8 days), TNFα strongly potentiates interleukin-3 (IL-3) and granulocyte-macrophage-CSF (GM-CSF)-induced growth of CD34⁺ HPC, while it has no proliferative effect per se. Within 8 days, the number of viable cells obtained in TNFα-supplemented cultures is threefold higher than in cultures carried out with IL-3 or GM-CSF alone. Secondary liquid cultures showed that the potentiating effect of TNFα on IL-3-induced proliferation of CD34⁺ HPC does not result from an IL-3-dependent generation of TNFα responsive cells. Limiting dilution analysis indicates that TNFα increases both the frequency of IL-3 responding cells and the average size of the IL-3-dependent clones. The potentiating effect of TNFα on IL-3- and GM-CSF-dependent growth of CD34⁺ HPC is also observed in day 7 colony assays. Under these short-term culture conditions, TNFα does not appear to accelerate cell maturation as a precursor morphology is retained. Finally, TNFα inhibits the relatively weak growth-promoting effect of granulocyte-CSF (G-CSF), which acts on a more committed subpopulation of CD34⁺ HPC different from that recruited by IL-3 and GM-CSF. TNFβ displays the same modulatory effects as TNFα. Thus, TNFs appear to enhance the early stages of myelopoiesis.