CD103 is a member of the integrin series of adhesion molecules. This antigen defines a developmentally important subset of T cells, namely mucosal T cells including all IEL (intraepithelial lymphocytes) and ~20% of lamina propria T cells. Expression of CD103 is more restricted outside these mucosal organs appearing at lower levels on T cell subsets of the lymph node, dendritic epidermis, and periphery. In non-epithelial CD103+ T cells there is a bias toward expression on CD8+ cells. The 2E7 monoclonal antibody exhibits antigen immunoprecipitation patterns similar if not identical to the rat anti-mouse CD103 monoclonal antibody M290. 2E7 is reported to have signal-inducing activity in a redirected lysis assay and to costimulate IEL and CD8+ lymph node cells in conjunction with anti-TCR in vitro which runs counter to the usual inhibition of CTL by anti-integrins. This costimulatory activity is also shared by the monoclonal antibody M290. The property of costimulation is not unique to the mouse since the combination of anti-human αIEL plus anti-TCR will produce similar proliferation in human cells with the exception that CD8+ cells are not stimulated.