Damage to the vasculature contributes significantly to the overall pathologic consequences of acute radiation exposure. Although at least part of this damage may be accountable to inflammatory reactions involving leukocyte/endothelial interactions, little is known of post-irradiation endothelial expression of the immunologically relevant surface molecules which are intimately involved in these processes. To address this issue, we have employed immunofluorescence flow cytometry to examine the effects ofγ-radiation on the constitutive and cytokine-induced expression of HLA class I, HLA class II, and intercellular adhesion molecule-1 (ICAM-1) by cultured human endothelial cells (EC). EC were -γ-irradiated (137Cesium, 1.2 Gray [GyJ/min) in confluent monolayer at doses up to 100 Gy, treated with interferon7 (IFNγ, 200 IU/ml, 72 hr), tumor necrosis factorα(TNFα, 40 IU/ml, 24 hr), or incubated in cytokine-free medium, then harvested and reacted with fluorescein-labelled monoclonal antibodies with specificities for HLA class I, HLA class II (DR), or ICAM-1. Quantitative fluorescence flow cytometric analysis demonstrated that neither constitutive expression nor cytokine-induced upregulation of any of these surface molecules was attenuated following even the highest radiation doses (100 Gy). Similarly, induction of endothelial HLA DR expression was observed in transwell cocultures when irradiated EC were exposed to cytokines liberated by activated T cells. In contrast, cell division was completely arrested by doses in the range of 6–10 Gy, as determined by standard clonogenic assay, and D0values for EC lines used in these experiments (1.0–1.8 Gy) were consistent with those commonly reported in the literature. These studies indicate that the constitutive endothelial expression of immunologically important surface molecules, as well as the cytokine responsiveness of these cells, represent highly radioresistant functions. This suggests that endothelial cells can retain their ability to participate interactively in the inflammatory response following substantial acute radiation exposure, and supports the hypothesis that endothelial cells provide an immune/inflammatory component in the overall radiation-induced pathologic response.