Abstract: Objective To study the effect of stromal cell-derived factor-1（SDF-1）/CXCR4 on the proliferation of CD34+ hematopoietic stem/progenitor cells（HSPCs） supported by bone marrow mesenchymal stem cells（MSCs）． Methods Rat bone marrow MSCs were plated as feeder layer in long-term cultures（LTC） with bone marrow CD34+ cells in vitro．Cultures were supplemented weekly with SDF-1，anti-SDF-1 antibody，or anti-CXCR4 antibody separately for 5 weeks．The hematopoietic-supporting activity was evaluated through the numbers of CD34+ cells and colony forming cell（CFC）．To assess the effect of SDF-1/CXCR4 on CD34+ cell proliferation cycle，killing assays were carried out and proliferation indexes were calculated．Expressions of SDF-1 and CXCR4 in MSCs and CD34+ cells were determined by flow cytometry．SDF-1 in MSC- and CD34+ cell-conditioned medium was also measured by ELISA． Results The numbers of CD34+ cells and CFC and proliferation indexes were significantly increased in treatment with SDF-1（P<0.01），and were significantly decreased in treatment with anti-SDF-1 antibody（P<0.05）and anti-CXCR4 antibody（P<0.01）．CXCR4 presents on the surface of CD34+ cells，but not on the surface of MSCs．Intracellular production of SDF-1(iSDF-1) was detected in MSCs，but not in CD34+ cells．SDF-1 was detected in MSC-conditioned medium，but not in CD34+ cell-conditioned medium． Conclusion SDF-1/CXCR4 play an important role in the hematopoietic-supporting activity of MSCs．