Experimental data in this study demonstrate that integrin α_vβ₃ is fundamentally involved in the maturation of blood vessels during embryonic neovascularization (vasculogenesis). Integrin α_vβ₃ was specifically expressed on the surface of angioblasts during vessel development in quail embryos and vitronectin, a ligand for α_vβ₃, localized to the basal surface of these cells. More importantly, microinjec tion of the anti-α_vβ₃ monoclonal antibody, LM609, disrupted the normal pattern of vascular development. After exposure to LM609 the angioblasts in experimental embryos appeared as clusters of rounded cells lacking normal cellular protrusions. This led to disruption of lumen formation and abnormal vessel patterning. These findings demonstrate that during vasculogenesis ligation of integrin α_vβ₃ on the surface of primordial endothelial cells is critical for the differentiation and maturation of blood vessels. Similar studies on chicken chorioallantoic membrane showed that LM609 blocks angiogenesis. Together the two studies suggest that integrin α_vβ₃ plays a role in neovascularization of tissues.