Abstract: The chemokine stroma-derived factor-1 (SDF-1) is produced within the bone marrow and mediates chemokinesis and chemotaxis on a variety of cell types that express the CXCR4 receptor. SDF-1-responsive cell types include monocytes and macrophages, B and T lymphocytes, platelets and megakaryocytes, and CD34+ cells, including both hematopoietic progenitors and stem cells. We have used intravenous injection of a replication-incompetent adenovector expressing the SDF-1 gene to elevate serum levels of SDF-1 in Balb/c and SCID mice. Within 3 to 5 days there was a marked leukocytosis, predominantly involving monocytes, and a three-fold increase in platelets. In addition, AdSDF-1 mobilized CFU-GM, CFU-s, and cells with long-term repopulating potential. We have identified a bone marrow-derived, circulating endothelial stem cell characterized by expression of the VEGFR2 (Flk-1/KDR). This cell exhibits a chemotactic and chemokinetic response to SDF-1 and VEGF. We have elevated serum levels of VEGF165 using intravenous adenovector gene delivery and compared this to an adenovector expressing angiopoietin-1 alone or in combination with VEGF. VEGF elevation was associated with rapid mobilization of hematopoietic stem and progenitor cells and a population of Flk-1-positive endothelial progenitors. In contrast angiopoietin induced a delayed mobilization of endothelial and hematopoietic progenitors. The combination of VEGF and angiopoietin produced a more prolonged elevation of these progenitors in the circulation with increased proliferation of capillaries and expansion of sinusoidal spaces in the marrow.