T cells expressing the gd receptor are the major lymphocyte type represented in intraepithelial layers of various organs in the mouse. The gd cells found in distinct anatomical locations express distinct restricted subsets of Vg and Vd genes. Cells in each site are probably derived from progressive intrathymic waves of gd cells expressing these restricted V gene subsets. These features of the cells suggest a role in monitoring epithelial layers for distinctive ligands, and recent studies implicate mycobacterial "stress proteins' as antigens for g/d T cells. Thus gd T cells may represent an arm of the immune system devoted to elimination of infected, transformed or otherwise stressed autologous epithelial cells, based on recognition of induced stress proteins. They may also be devoted to mycobacterial immunity by virtue of specificity for mycobacterial stress proteins and other antigens.