Damage to blood vessels caused by ionizing radiation is considered to be an important dose limiting factor for late effects in many organs. However, the radiation response of the endothelial cells which line the vasculature has not been well-documented, particularly for human endothelial cells. In the present study, human endothelial cells were obtained from fresh umbilical cords and cultured in monolayer. Immunofluorescent staining of factor VIII antigen was used to verify the endothelial nature of the cultured cells. The cells were irradiated with Cs-137 rays (1.35 Gy/min) in plateau phase to determine radiation sensitivity and ability to repair potentially lethal damage (PLD) and sub-lethal damage (SLD). The endothelial cells demonstrated moderate radiosensitivity that varied slightly between different cords. As demonstrated by delayed plating experiments, PLD repair ability was substantial, with repair factors of 0.70-0.80. SLD repair capability, as demonstrated by split dose experiments, was relatively modest. A survival enhancement of 2.0-2.2, for example, was observed when 8 Gy single dose survival was compared to two 4 Gy doses. In terms of PLD and SLD repair as well as initial slope (alpha) the endothelial cells were similar to normal human lung and skin fibroblasts previously studied. Compared to malignant cell lines, PLD repair was generally larger whereas the initial slope (alpha) was intermediate-steeper than the radioresistant tumor types but shallower than the more sensitive tumor derived cells.