Cleaved forms of soluble urokinase receptor (c-suPAR) have been detected in body fluids from patients affected by various tumors. We recently reported increased c-suPAR levels in sera of healthy donors during granulocyte colony-stimulating factor (G-CSF)–induced mobilization of CD34⁺ hematopoietic stem cells (HSC). In vitro, c-suPAR or its derived chemotactic peptide (uPAR_84-95) stimulated migration of human CD34⁺ HSCs and inactivated CXCR4, the chemokine receptor primarily responsible for HSC retention in bone marrow. These results suggested that c-suPAR could potentially contribute to regulate HSC trafficking from and to bone marrow. Therefore, we investigated uPAR_84-95 effects on mobilization of mouse CD34⁺ hematopoietic stem/progenitor cells (HSC/HPC). We first showed that uPAR_84-95 stimulated in vitro dose-dependent migration of mouse CD34⁺ M1 leukemia cells and inactivated murine CXCR4. uPAR_84-95 capability to induce mouse HSC/HPC release from bone marrow and migration into the circulation was then investigated in vivo. uPAR_84-95 i.p. administration induced rapid leukocytosis, which was associated with an increase in peripheral blood CD34⁺ HSCs/HPCs. In vitro colony assays confirmed that uPAR_84-95 mobilized hematopoietic progenitors, showing an absolute increase in circulating colony-forming cells. uPAR_84-95 mobilizing activity was comparable to that of G-CSF; however, neither synergistic nor additive effect was observed in combining the two molecules. These findings show for the first time in vivo biological effects of c-suPAR. Its capability to mobilize HSCs suggests potential clinical applications in HSC transplantation. (Cancer Res 2006; 66(22): 10885-90)