Background: Limitation of intracoronary infusion of stem cells is the insufficient rate to ischemic tissue (less than 5%). In order to maximize its therapeutic applicability, the vasculogenic potential of stem cells needs to be further increased. We introduce the concept of ���priming��� mobilized peripheral blood stem cells (PBSCs) with G-CSF using Angiopoietin-1(Ang-1). Methods and Results: Ang-1 was administered as a priming agent because it was found that PBSCs highly express Tie2, Ang-1 receptor [Percentage of Tie2-positive cells, 19.2짹3% vs. 1.2짹0.2%, p<0.01 for PBSCs vs peripheral blood mononuclear cells]. We investigated whether the effect of Ang-1 on PBSCs differentiation is mediated via the transcriptional regulation of ets-1. The gene expression of ets-1 increased in a time dependent manner after Ang-1 stimulation (400ng/ml) in PBSCs. Compared with vehicle, treatment with Ang-1 for 4 hours resulted in stronger expression of endothelial markers, such as, CD31 and VE-cadherin, which were significantly reduced with antisense ets-1 oligodeoxynucleotides (ODN). These results were also confirmed at the protein level by flow cytometry analysis. We postulated that integrins that contain an ets-1 binding motif in their promoter region may be activated by Ang-1, such as 慣4棺1 and 慣5棺1 integrins. RT-PCR showed increased mRNA expression of 慣4, 慣5, and 棺1 integrins after Ang-1 stimulation, and its attenuation after transfection with antisense ets-1 ODN. These results were also confirmed at the protein level by flow cytometry analysis. To correlate these findings with functional changes in PBSCs, we performed in vitro adhesion assays. Ang-1 stimulation increased PBSCs adhesion to HUVECs and fibronectin, while pretreatment with anti-hTie2 or anti-integrin antibody prior to Ang-1 stimulation abrogated these results. Finally, priming PBSCs with Ang-1 resulted in improved single pass engraftment in ischemic rabbit ear after intra-artery delivery, and enhanced neovascularization in a nude mouse ischemic hindlimb model. Conclusion: These results suggest that priming PBSCs with Ang-1 may enhance the efficacy of transplanted PBSCs in terms of increasing neovascularization in ischemic tissue.