We have discovered that endothelial progenitor cells (EPC) are particularly suitable for use in a sensitive assay for antiangiogenic factors. We have found that EPC mobilization and differentiation is greatly inhibited by antiangiogenic factors as evidenced in vivo by VEGF inducing a massive mobilization of EPC into the blood circulation which effect is significantly inhibited by endostatin treatment, and, in vitro, human blood-derived EPC forming adherent colonies, which colonies, in the presence of angiogenic factors, give rise to differentiated EC, and which differentiation is disrupted and cell growth is inhibited in the presence of angiostatin and endostatin.