The vast majority of mature T lymphocytes in the peripheral blood and lymphoid organs use the CD3-associated alpha, beta T-cell receptor (TCR) heterodimer for antigen recognition. A second class of TCRs consists of disulphide-linked gamma and delta proteins that are also CD3-associated. A subset of early CD3+ fetal and adult CD4- 8- thymocytes express gamma, delta TCRs before alpha, beta TCRs are detectable. In addition, a minor (1-5%) subpopulation of peripheral T lymphocytes, and some spleen cells from nude mice express gamma, delta TCRs. Notably, dendritic epidermal cells have also been shown to express gamma, delta TCRs. All of these populations lack CD4 and CD8 molecules. We now report that most mature T cells residing in the murine intestinal epithelium express CD3-associated TCRs composed of gamma-chains disulphide-linked to a protein resembling the delta-chain. The striking feature of these intraepithelial lymphocytes (IEL) was that they were exclusively CD4-8+. In addition, approximately half of CD3-bearing IEL lacked detectable Thy-1 on the cell surface, which is unprecedented for murine T cells. In contrast to other CD8+ peripheral T cells, freshly isolated IEL could be induced to display cytolytic activity by engaging the CD3 molecule, indicating that activation had occurred in vivo. Thus, CD8+ IEL are a phenotypically diverse and anatomically restricted population of lymphocytes that use gamma-chain containing heterodimers for antigen recognition.