An inhibitor has been identified in the conditioned medium of hamster cells and hamster-human hybrids that suppresses neovascularization in vivo in the rat cornea. Inhibitory activity was tightly linked to the presence of an active cancer suppressor gene in transformants and revertants, in segregating hybrids, and in temperature-limited transformants. It copurified with a~ 140 kd glycoprotein. Polyclonal antiserum raised against the purified preparation recognized a 140 kd protein in conditioned medium and was able to adsorb out all antiangiogenic activity. These results define the control of the activity of an inhibitor of neovascularization as one function of the cancer suppressor gene active in BHK21M13 cells and simultaneously identify a new inhibitor of angiogenesis, a process vital to the growth of solid tumors.