The human TCR is composed of the Ti alpha beta heterodimer in association with the CD3 chains CD3 gamma delta epsilon zeta 2. Another chain, referred to as CD3 omega, has recently been described in T cells. CD3 omega is an intracellular protein transiently associated with the CD3 complex during the assembly of the TCR in the endoplasmic reticulum (ER) and it is not expressed on the cell surface. The function of CD3 omega is unknown but it has been suggested that it plays an important role in the assembly of the TCR. We have studied the possible function of CD3 omega in the human leukemic T-cell line Jurkat and different variants of this cell line. Cells were metabolically labeled, subjected to lysis, immunoprecipitated, and analyzed by SDS-PAGE. The results indicate that: 1) CD3 omega associates primarily with the CD3 delta epsilon complex; 2) CD3 omega is not associated with single Ti alpha or Ti beta chains, but is present in complexes composed of both the CD3 and the Ti chains; 3) CD3 omega is part of the complete, intracellular receptor complex Ti alpha beta/CD3 gamma epsilon delta omega zeta 2; and 4) CD3 omega dissociates from the Ti/CD3 complex in the ER before maturation of the Ti alpha beta heterodimer. On the basis of these results, we propose a model for the assembly and subunit stoichiometry of the TCR complex which includes the participation of the CD3 omega chain.