The T-cell antigen receptor (TCR) consists of a glycoprotein heterodimer (α/β or γ/δ) which is non-covalently associated with at least four or five invariant polypeptides (CD3 γ,δ,ε, ;ξ and η). In T-cell variants lacking TCR α,β or ξ, it has been shown that incomplete TCR/CD3 complexes are retained within the cell. To examine requirements for cell surface expression of TCR/CD3, we transfected COS monkey kidney cells with cDNAs encoding TCR α,β and CD3 γ, δ, ε and ξ. We report that cell surface appearance of TCR/CD3 on COS cells requires coordinate expression of all six proteins. In the absence of the ξ chain, subcomplexes comprising from two to five chains were readily demonstrable In COS cells, but they failed to reach the cell surface or to acquire N-llnked oligosaccharide side chains indicating failure to reach the medial Golgl. Pulse-chase, metabolic labelling of transfected COS cells showed that three chains (CD3 γ, CD3 ε, and ξ) were stable while three (TCR α, TCR β and CD3 δ) were rapidly degraded. In two- and three-chain co-transfections specific intracellular subcomplexes were formed between TCR α and CD3 γ, TCR α and CD3 δ, or TCR β and CD3 ε. Binary subcomplexes having at least one stable chain (CD3 ε–TCR β) were stable while one formed by two unstable chains (TCR α–CD3 δ) was still degraded. Assembly of the TCR/CD3 complex in COS cells thus appears centered around the metabolically stable CD3 γ and CD3 ε proteins. Site-specific mutations of the negatively-charged transmembrane amino acid of residues of the CD3 chains to alanines served to either abolish (for TCR α–CD3 δ and TCR β–CD3 ε) or diminish (for TCR α–CD3 γ) these TCR-CD3 interactions. These mutations had no effect, however, on CD3–CD3 Interactions or upon synthesis, metabolism, or intracellular distributions of the CD3 proteins. The transmembrane domains of CD3 γ, δ, and ε thus appear to play a major role in associations of CD3 with TCR chains.