Abstract-Collateral growth is characterized by macrophage accumulation, suggesting an important role of circulating cells. To study origin and function of macrophages during arteriogenesis, we related the extent of macrophage accumulation to vascular proliferation and investigated the fate of fluorescently (CMFDA) labeled blood cells that were injected at the time of femoral artery occlusion. The effect of bone marrow depletion via cyclophosphamide before femoral artery occlusion on collateral proliferation and macrophage accumulation was studied, and we looked for the presence of bone marrow-derived stem cells in the vicinity of growing collateral vessels. Finally, we investigated the arteriogenic effect of macrophage activation via MCP-1 in bone marrow-depleted animals. Maximal macrophage accumulation occurred during the first 3 days after femoral artery occlusion and paralleled the extent of vascular proliferation. Fluorescently labeled leukocytes homed to spleen and wound but they were absent in proliferating collateral arteries during maximal macrophage accumulation. Depletion of circulating cells did neither affect macrophage accumulation nor collateral growth. Staining of monocyte-depleted animals for BrdUrd and ED2, ␣SMA, or VE-Cadherin demonstrated local proliferation of macrophages and vascular cells, whereas C-Kit, SSEA1, or Thy1-positive bone marrow-derived stem cells were not detectable. Enhancement of macrophage accumulation via MCP-1 was independent of circulating monocytes and promoted arteriogenesis in the absence of direct effects on vascular cells. We propose that the initial phase of vascular growth is characterized by local proliferation of tissue resident precursors rather than by migration of blood born cells.