The formation of new blood vessels is critical to tumor growth and metastasis and since endothelial cells are the principal cell type comprising such blood vessels, we carried out experiments to test whether the growth of endothelial cells in vitro can be inhibited by low doses of combinations of cytokines. We show that growth of mouse aortic endothelial cells can be inhibited in vitro by combinations of cytokines that, administered singly even at ten times higher concentrations, have only marginal effects. In particular, interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma), acting synergistically at low, subthreshold concentrations, are able to completely inhibit the growth of endothelial cells in vitro.