Tumor necrosis factor (TNF) is cytotoxic for many tumoral cell lines, whereas normal cells generally are considered resistant to this action. This study shows that this cytokine causes massive death of bovine endothelial cells in primary culture in a concentration- and time-dependent manner. Dying cells exhibit all the ultrastructural changes and the inter-nucleosome cleavage of DNA associated with apoptosis or 'programmed cell death.' This is the first report clearly showing a direct toxicity of TNF on endothelial cells and demonstrating that this results from the induction of the program of apoptotic death. Our observation raises the possibility that hemorrhagic necrosis in vivo, after treatment with TNF, might involve a direct cytocidal action on endothelial cells of the tumor neovasculature.