We report the results of a multidisciplinary study on the inhibitory effect of a snake venom disintegrin, contortrostatin, a 13.5 kDa homodimeric protein isolated from Agkistrodon contortrix contortrix (southern copperhead) venom, on breast cancer progression. We demonstrate that contortrostatin binds to integrins and blocks the adhesion of human breast cancer cells (MDA-MB-435) to extracellular matrix (ECM) proteins including fibronectin and vitronectin, but it has no effect on adhesion of the cells to laminin and Matrigel. Contortrostatin also prevents invasion of MDA-MB-435 cells through an artificial Matrigel basement membrane. Daily local injection of contortrostatin (5 microg per mouse per day) into MDA-MB-435 tumor masses in an orthotopic xenograft nude mouse model inhibits growth of the tumor by 74% (p = 0.0164). More importantly, it reduces the number of pulmonary macro-metastasis of the breast cancer by 68% (p < 0.001), and micro-metastasis by 62.4% (p < 0.001). Contortrostatin is not cytotoxic to cancer cells, and does not inhibit proliferation of the breast cancer cells in vitro. However, contortrostatin inhibits angiogenesis induced by the breast cancer, as shown by immunohistochemical quantitation of the vascular endothelial cells in tumor tissue removed from the nude mice. We have identified alpha(v)beta3, an important integrin mediating cell motility and tumor invasion, as one of the binding sites of contortrostatin on MDA-MB-435 cells. We conclude that contortrostatin blocks alpha(v)beta3, and perhaps other integrins, and thus inhibits in vivo progression.