Most gut intraepithelial cells (IEL) of the mouse are T cells that bear CD8 molecules, present either as alpha-beta chain heterodimers (CD8 beta+) or as alpha chain homodimers (CD8 beta-). All CD8 beta+ IEL bear alpha/beta T cell receptors (TCR); CD8 beta- IEL bear either alpha/beta or gamma/delta TCR and are considered to be a thymus-independent (TI) population, probably arising locally from a small fraction of CD3- IEL containing the recombinant activating gene RAG proteins. Here we report that TI CD8 beta- IEL, whether bearing alpha/beta or gamma/delta TCR, contain, in normal mice, mRNAs for both zeta and Fc epsilon RI gamma chains. These chains are present in their CD3-TCR complexes as homo- or heterodimers. In contrast, only zeta chain mRNA and homodimers are found in gut CD8 alpha/beta+ IEL and in peripheral T lymphocytes. Intestinal CD3- precursor cells contain only gamma chain, and CD3- IL-2R+ thymocyte precursors only zeta chain mRNAs. Only very primitive thymocyte precursors contain detectable gamma chain mRNA, and it thus appears that Fc epsilon RI gamma chain use is switched off at a very early stage during thymocyte differentiation. Thus, T cell differentiation in the gut epithelium differs from that occurring in the thymus, from which CD8 beta+ IEL appear to derive. Use of different TCR transducing modules and CD8 accessory molecules between the TI and the thymus-derived T cell populations provides an explanation for their difference in reactivity to antigenic stimulations and thus in selection of repertoires.