The radiation response of cultured rabbit aortic endothelial cells was measured using colony formation to determine survival. The dose survival curve was qualitatively similar to those reported for other cell types. At doses of 400 rad and greater, the curve was an exponential function of dose with a D0 of 120 rad and an n of 7. Exponentially growing endothelial cell cultures recovered from sublethal damage between two doses of radiation. Plateau phase cultures recovered from potentially lethal damage when incubated for 6 hr between irradiation and assay.