Purified primitive progenitor/stem cells from bone marrow represent likely target populations for ex vivo expansion of stem cells to be used in high-dose chemotherapy or gene therapy. Whereas such primitive progenitor cells require combined stimulation by multiple cytokines for growth, some cytokines selectively promote viability rather than growth when acting individually. We investigated here for the first time the direct effects of cytokines on survival of primitive CD34⁺CD38⁻ human bone marrow progenitor cells at the single-cell level. Interleukin-3 (IL-3) and the ligands for c-kit (KL) and flt3 (FL) had direct and selective viability-promoting effects on a small fraction of CD34⁺CD38⁻ but not CD34⁺CD38⁺ progenitor cells. Interestingly, the recently cloned thrombopoietin (Tpo), although stimulating little growth, kept most CD34⁺CD38⁻ progenitors viable after prolonged culture, maintaining twofold and fourfold more progenitors viable than KL and IL-3, respectively. A high fraction of these progenitors had a combined myeloid and erythroid differentiation potential, as well as capacity for prolonged production of progenitor cells under stroma-independent conditions. In addition, Tpo promoted viability of CD34⁺CD38⁻ long-term culture-initiating cells, further supporting the idea that Tpo promotes viability of primitive human progenitor cells. Finally, Tpo suppressed apoptosis of CD34⁺CD38⁻ cells in culture. Thus, the present studies show a novel effect of Tpo, implicating a potential role of this cytokine in maintaining quiescent primitive human progenitor cells viable.