Somatic stem cells are largely quiescent in spite of their considerable proliferative potential. Transforming growth factor-(beta)1 (TGF-(beta)1) appears to be a good candidate for controlling this quiescence. Indeed, various mutations in the TGF-beta signalling pathway are responsible for neoplasic proliferation of primitive stem/progenitor cells in human tissues of various origins. In hemopoietic single cell culture assays, blocking autocrine and endogeneous TGF-(beta)1 triggers the cell cycling of high proliferative potential undifferenciated stem/progenitor cells. However, it has never been demonstrated whether TGF-(beta)1 has an apoptotic effect or a differentiating effect on these primitive cells, as already described for more mature cells. Using single cell experiments both in liquid or semi-solid culture assays and dye tracking experiments by flow cytometry, we demonstrate that low, physiological concentrations of TGF-(beta)1, which specifically maintain primitive human hemopoietic stem/progenitor cells in quiescence, have a reversible effect and do not induce apoptosis. We moreover demonstrate that these low concentrations prevent the rapid loss of the mucin-like protein CD34, a most common marker of immature hematopoietic stem/progenitor cells, which is progressively lost during differentiation. TGF-(beta)1 not only up-modulated the CD34 antigen before S phase entry but also maintained a high level of CD34 expression on cells which had escaped cell cycle inhibition, suggesting that proliferation inhibition and differentiation control by TGF-(beta)1 may be independent. These data provide additional evidence that TGF-(beta)1 acts as a key physiological factor ensuring the maintenance of a stem cell reserve.