Chimeric Ag receptors (CARs) expressed in T cells permit the redirected lysis of tumor cells in an MHC-unrestricted manner. In the Jurkat T cell model system, expression of a carcinoembryonic Ag-specific CD3ζ CAR (MFEζ) resulted in an increased sensitivity of the transduced Jurkat cell to generate cytokines when stimulated through the endogenous TCR complex. This effect was driven through two key characteristics of the MFEζ CAR: 1) receptor dimerization and 2) the interaction of the CAR with the endogenous TCR complex. Mutations of the CAR transmembrane domain that abrogated these interactions resulted in a reduced functional capacity of the MFEζ CAR to respond to carcinoembryonic Ag protein Ag. Taken together, these results indicate that CARs containing the CD3ζ transmembrane domain can form a complex with the endogenous TCR that may be beneficial for optimal T cell activation. This observation has potential implications for the future design of CARs for cancer therapy.