The chemokine stromal cell-derived factor-1 (SDF-1), and its receptor, CXCR-4, have been implicated in the homing and mobilization of human CD34⁺ cells. We show here that SDF-1 may also be involved in hematopoiesis, promoting the proliferation of human CD34⁺ cells purified from normal adult peripheral blood (PB). CXCR-4 was expressed on PB CD34⁺ cells. The amount of CXCR-4 on PB CD34⁺ cells was 10 times higher when CD34⁺ cells were purified following overnight incubation. CXCR-4 overexpression was correlated with a primitive PB CD34⁺ cell subset defined by a CD34^high CD38^lowCD71^lowc-Kit^lowThy-1⁺antigenic profile. The functional significance of CXCR-4 expression was ascertained by assessing the promoting effect of SDF-1 on cell cycle, proliferation, and colony formation. SDF-1 alone increased the percentage of CD34⁺ cells in the S+G₂/M phases and sustained their survival. In synergy with cytokines, SDF-1 increased PB CD34⁺ and CD34^highCD38^low cell expansion and colony formation. SDF-1 also stimulated the growth of colonies derived from primitive progenitors released from quiescence by anti–TGF-β treatment. Thus, our results shed new light on the potential role of this chemokine in the stem cell engraftment process, which involves migration, adhesion, and proliferation. Furthermore, both adhesion-induced CXCR-4 overexpression and SDF-1 stimulating activity may be of clinical relevance for improving cell therapy settings in stem cell transplantation.