The T cell antigen receptor (TCR) and pre-TCR complexes are composed of multiple signal-transducing subunits (CD3γ, CD3δ, CD3ε, and ζ) that each contain one or more copies of a semiconserved functional motif, the immunoreceptor tyrosine-based activation motif (ITAM). Although biochemical studies indicate that individual TCR-ITAMs may bind selectively or with different affinity to various effector molecules, data from other experiments suggest that at least some ITAMs are functionally equivalent. In this study, we examined the role of CD3ε ITAM-mediated signals in T cell development by genetically reconstituting CD3ε-deficient mice with transgenes encoding either wild-type or ITAM-mutant (signaling defective) forms of the protein. The results demonstrate that signals transduced by CD3ε are not specifically required for T cell maturation but instead contribute quantitatively to TCR signaling in a manner similar to that previously observed for ζ chain. Unexpectedly, analysis of TCR-transgenic/CD3ε-mutant mice reveals a potential role for CD3ε signals in T cell survival.