Stromal cell-derived factor-1 (SDF-1) is a CXC chemokine produced by stromal cells that acts as a chemoattractant for human CD34 + progenitor cells. We investigated the expression of CXCR4, the receptor for SDF-1, on CD34 + cells from different hematopoietic sites and developmental stages. CXCR4 was detected by flow cytometry on 37 % of fetal bone marrow (BM) [gestation weeks (gw) 14 -23] and 40 % of adult BM CD34 + cells. Interestingly, in fetal liver CD34 + cells, CXCR4 was expressed at lower levels at later stages (9 %, gw 20 -23) compared to early stages of development (39 %, gw 7.5 -18), suggesting a developmentrelated change in the migratory capacity of progenitors. CXCR4 was detected at similar levels on both phenotypically primitive and committed progenitors from fetal and adult sites. However, B cell lineage progenitor and precursor cells expressed CXCR4 at the highest density (80 % of BM CD34 + /CD10 + pro-B cells are CXCR4 + ). CXCR4 was also expressed in the fetal thymus in early T cell precursors and found to be down-regulated during T cell maturation. Finally, we found that stem cell factor, alone or in combination with other cytokines, can up-modulate CXCR4 expression on CD34 + cells by three-to fourfold. In conclusion, our results suggest that CXCR4 may play an important role in the local and systemic trafficking of human CD34 + cells as well as in human B lymphopoiesis and that its expression can be modulated by cytokines.