Factors defining the previously established differences in susceptibility of endothelial cells to the tumor-necrotizing agents, tumor necrosis factor (TNF) and lipid A, were investigated. Venous and arterial endothelial cells isolated from bovine and human umbilical cords showed large differences in proliferation rate and susceptibility to TNF and lipid A as measured by [3H]thymidine incorporation. The faster proliferating cells appeared more strongly inhibited by the agents. Also, using different isolates of human venous endothelial cells a similar correlation between proliferation rate and degree of inhibition was found. Growth stimulation of human venous cells with endothelial cell growth factor, but not the growth factor combined with heparin, increased the inhibitory action of the agents. Influence of the serum source was investigated by culturing human and bovine cells in the presence of human or bovine serum. Lipid A, but not TNF, was more inhibitory to cells of both species when cultured in bovine serum as compared to human serum. Supernatant of cultured tumor cells and serum from tumor-bearing mice increased the inhibitory effects of the agents as well. Data show that the action of the tumor-necrotizing agents on endothelial cells is defined by various factors, such as origin of the cells and culture conditions. In general, factors promoting cell proliferation tended to increase the susceptibility of the cells to the agents. The reported high vulnerability of tumors, wound tissue and placenta to induction of hemorrhage by these agents may be partially due to an enhanced sensitivity of the fast proliferating endothelium in these tissues.