Angiogenesis is fundamental to a variety of physiological and pathological processes. While a number of factors have been identified that induce neovascularization, it is becoming increasingly apparent that endogenous angiostatic factors may play an important role in the regulation of angiogenesis during wound repair, chronic inflammation, and growth of solid tumors. In this study, we demonstrate the novel finding that IP-10, a member of the C-X-C chemokine family, is a potent inhibitor of both IL-8 and bFGF-induced angiogenic activity using in vitro and in vivo assays of angiogenesis. These findings support the contention that IP-10 may be a pivotal cytokine in the regulation of neovascularization.