Abstract: The present invention is directed to a method of inhibiting tumor formation in a cancer patient at a site remote from sites of prior tumor formation and to a method of preventing metastases. These methods involve administering to the cancer patient an inhibitor of vascular endothelial growth factor receptor 1+ bone marrow-derived cells under conditions effective either to inhibit tumor formation in the cancer patient at a site remote from sites of prior tumor formation or to prevent metastases. Candidate compounds useful for such purposes can be screened depending on whether they bind to vascular endothelial growth factor receptor 1+ bone marrow-derived cells. Metastases in a cancer patient can be monitored by evaluating a patient sample for detection and quantification of vascular endothelial growth factor receptor 1+ bone marrow-derived cells and comparing the level of vascular endothelial growth factor receptor 1+ bone marrow-derived cells to prior levels.