The monoclonal antibody 5H10.27 reacts with the a5 chain (VLA-5) of the CD49e/CD29 heterodimeric fibronectin receptor. It is expressed on thymocytes, activated T cells, mast cells, and a variety of mouse cell lines. Soluble 5H10.27 antibody inhibits VLA-5-mediated functions in vitro. Immobilized 5H10.27 has also been demonstrated to costimulate the proliferative response of CD8+ T cells to plate-bound anti-CD3 monoclonal antibody.