The H9.2B8 antibody reacts with the 140-kDa integrin αv chain (CD51). Heterodimers of CD51 with several integrin β chains function as receptors for extracellular matrix (ECM) proteins. CD51/CD61 (αvβ3 integrin, vitronectin receptor) mediates adhesion to fibronectin, fibrinogen, vitronectin, thrombspondin, von Willebrand factor, and CD31 (PECAM-1). It is expressed on activated T lymphocytes, polymorphonuclear granulocytes, blastocysts, and osteoclasts. We have been unable to detect CD51 on mouse platelets using either mAb H9.2B8 or RMV-7. CD51 also forms heterodimers with integrin β1 (CD29) and β5, β6, and α8 chains. αv integrins have diverse functions in development and homeostasis. H9.2B8 antibody binds to an epitope of CD51 near or at the receptor-ligand binding site(s), as evidenced by its ability to synergistically participate in blocking of the adherence of vitronectin receptor-bearing cells to ECM proteins. This hamster mAb to a mouse leukocyte antigen does not cross-react with rat leukocytes.