Recent studies have demonstrated that the CD3-ζ subunit of the T cell antigen receptor (TCR) complex is involved in signal transduction. However, the function of the remaining invariant subunits, CD3-γ, -δ, and , is still poorly understood. To examine their role in TCR function, we have constructed TCR/CD3 complexes devoid of functional ζ subunit and showed that they are still able to trigger the production of interleukin-2 in response to antigen or superantigen. These data, together with previous results, indicate that the TCR/CD3 complex is composed of at least two parallel transducing units, made of the γδ and ζ chains, respectively, Furthermore, the analysis of partially truncated ζ chains has led us to individualize a functional domain that may have constituted the building block of most of the transducing subunits associated with antigen receptors and some Fc receptors.