The 2C9.G2 monoclonal antibody specifically binds to the integrin β3 chain (CD61), which associates with the integrin αv chain (CD51) to form the vitronectin receptor, as well as the αIIb chain (CD41) to form the gpIIb/IIIa complex. Both receptors mediate adhesion to fibronectin, fibrinogen, vitronectin, thrombospondin, and von Willebrand factor. Leukocyte-endothelial adhesion is also mediated by the binding of αvβ3 integrin or vitronectin receptor to CD31 (PECAM-1). In addition, interaction of the αvβ3 integrin with its ligands regulates the L-type Ca2+ channel in vascular smooth muscle cells, possibly mediating vasodilatory responses to injury. Soluble and insoluble 2C9.G2 mAb mimics the effect of the natural ligands in smooth muscle cells from rat cremaster arterioles. Furthermore, osteopontin, also named Eta-1, is a cytokine that binds to αvβ3. CD61 is expressed on platelets, activated T lymphocytes, polymorphonuclear granulocytes, and blastocysts. Cross-reactivity of mAb 2C9.G2 to rat mast cells and platelets has been observed by flow cytometric analysis. mAb 2C9.G2 has been demonstrated to block binding of rat and mouse cells to fibronectin.