Transgenic mice carrying and expressing the human CD3 epsilon gene incorporate the corresponding protein product into T cell receptor (TCR)/CD3 complexes on thymocyte and T cell surfaces. The chimeric antigen receptors allow normal T cell development and selection of repertoires in vivo and are able to transduce activation signals in vitro. We have exploited the ability to distinguish mouse (m) and human (h)CD3 epsilon chains to analyze the stoichiometry of CD3 epsilon in transgenic mouse TCRs. Immunoprecipitation and fluorescence resonance energy transfer experiments demonstrate that such TCRs can contain both h- and mCD3 epsilon chains, implying that more than one CD3 epsilon subunit occurs per TCR. Antigen comodulation studies are consistent with a stochastic use of h- or mCD3 epsilon during receptor assembly, and further suggest a structure for the TCR/CD3 complex with two CD3 epsilon chains. The determination of CD3 epsilon subunit stoichiometry, together with existing biochemical data, allows the generation of a minimal model for the structure of the TCR and illustrates the potential value of the transgenic approach to the analysis of complex receptors.