This paper reports the discovery in the mouse of a new antigen found almost exclusively on the surface of lymphocytes residing in or immediately adjacent to the gut epithelium. The antigen was expressed by Lyt-2+ and L3T4+ cells but not by B cells or plasma cells and was present on almost all intraepithelial lymphocytes (IELs) in the gut. Only a very small proportion of cells in other lymphoid compartments expressed the antigen. Stimulation of IELs or other lymphocytes in vitro caused a decline in expression. Immunoprecipitation experiments showed the new antigen to be a molecular complex comprising two non-covalently linked chains (175 kDa and 136 kDa) and minor components (27 kDa and 25 kDa). The function of this complex is unknown but its structure has certain features in common with that of cell adhesion molecules and extracellular matrix receptors of the 'integrin' supergene family.