We develop an assay platform utilizing donor-specific endothelial progenitor cells as a proxy to test individual sensitivity to ionizing radiation (IR). IR is known to cause long-term complications. In our method, individual IR sensitivity is correlated to an in vitro response to radiation of endothelial progenitor cells, also called endothelial colony forming cells (ECFCs), which critically contribute to the healing of injured tissues. ECFC recruitment, proliferation, and differentiation have been proposed to be a key to tissue recovery after radiation-induced damage. Our assay platform will help to test individual susceptibility to radiation-mediated damage and enable the discovery of novel radioprotectors.