The zeta (ζ) chain plays a central role in T cell antigen receptor assembly and signal transduction. From previous work in murine T cell hybridomas we have inferred that the ζ subunit is limiting in receptor assembly. Partial receptors made in excess of ζ are assembled in the endoplasmic reticulum, transported through the Golgi, but then rapidly and efficiently degraded in lysosomes. ζ Would therefore seem to play a unique role in targeting receptors from the Golgi to the cell surface. To determine directly whether ζ limits receptor assembly we have reconstituted a ζ-deficient T cell line by transfection of the murine ζ cDNA. Transfection results in restoration of expression of surface T cell receptor. In addition, increasing ζ expression results in a commensurate increase in the survival of previously excess subunits. This is reflected in an increased surface expression of complete receptors. Finally, transfection of the ζ cDNA fails to produce detectable ζ-η heterodimers. The implications of these findings with regard to receptor assembly, and the relationship between ζ and η, are discussed.