Intestinal intraepithelial lymphocytes (iIEL) are a distinct subpopulation of T lymphocytes diffusely distributed in the epithelium. Because > 95% of these iIEL express the integrin alpha E beta 7, we reasoned that this integrin might mediate the localization of iIEL to the epithelium. We report that cultured iIEL bound to mucosal epithelial cells derived from either breast or intestine. This binding was dependent on the presence of divalent cations, and was blocked by mAb specific for two integrins, alpha E beta 7 and alpha L beta 2. After TGF-beta 1 treatment, cultured iIEL cell lines expressed alpha E beta 7 at levels similar to those found on iIEL in vivo. Under these conditions this integrin appeared to predominate in mediating the adhesion of iIEL to epithelial cells. Although alpha L beta 2 blocked the adhesion of iIEL to both endothelial and epithelial cells, alpha E beta 7 blocked binding only to epithelial cells suggesting that the ligand for alpha E beta 7 is not expressed on IL-1 beta-treated cultured endothelial cells. iIEL did not readily bind to the apical surface of confluent polarized epithelial cell monolayers. However, iIEL did bind to these cells when tight junctions were disrupted, a treatment that allows redistribution of proteins compartmentalized to the basolateral surface. This binding was also blocked by mAb to alpha E beta 7. In addition, in vitro coculture of iIEL with polarized epithelial cells resulted in basolateral localization of the iIEL. Thus, it is likely that expression of the alpha E beta 7 ligand is restricted to the basolateral membrane of polarized epithelial cells. Because alpha E beta 7 appears to be a predominant integrin on freshly isolated iIEL, we hypothesized that alpha E beta 7-mediated adhesion may be particularly important in cell-to-cell interactions between T cells and epithelial cells in vivo.