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Background: Sodium selenite is applied in tumor patients during chemo- or radiotherapy due to its cytoprotective effects. Aim of our study was to evaluate the effect of exposure with sodium selenite on proliferation of human endothelial and tumor cells after irradiation., Materials and Methods: We studied the proliferative activity of human umbilical vein endothelial cells in comparison to tumor cells of the HeLa, MIA Paca-2 and SiHa cell line after single-dose irradiation with 2 or 10 Gy and controls without irradiation. All cells had been exposed to different concentrations of sodium selenite prior to irradiation. Evaluation was done by BrdU-ELISA., Results: Exposure of human endothelial cells with sodium selenite concentrations > or = 100 micrograms/l led to an increase of BrdU proliferation index. This effect was markedly weaker in HeLa cells and not found in SiHa and MIA Paca-2., Conclusions: High concentrations of sodium selenite can counteract the decrease of proliferative activity caused by irradiation in human endothelial cells and thus exert a radioprotective effect on these cells. This effect was observed by far stronger in endothelial cells than in tumor cells, implying the possible clinical use of sodium selenite as a protective agent for normal tissue in radiotherapy.