Angiogenesis depends on the adhesive interactions of vascular cells. The adhesion receptor integrin α_vβ₃ was identified as a marker of angiogenic vascular tissue. Integrin α_vβ₃ was expressed on blood vessels in human wound granulation tissue but not in normal skin, and it showed a fourfold increase in expression during angiogenesis on the chick chorioallantoic membrane. In the latter assay, a monoclonal antibody to α_vβ₃ blocked angiogenesis induced by basic fibroblast growth factor, tumor necrosis factor-α, and human melanoma fragments but had no effect on preexisting vessels. These findings suggest that α_vβ₃ may be a useful therapeutic target for diseases characterized by neovascularization.