Kinetic features of TCR:MHC/peptide interactions dictate their outcome in vitro, some important parameters of which include the number of molecules engaged and the duration of engagement. We explored the in vivo significance of these findings in transgenic mice expressing TCRs in a quantitatively and temporally controlled manner. As anticipated, reduced TCR levels resulted in attenuated reactivity, but response thresholds were substantially lower than expected-at as low as 1/20th the normal TCR numbers and with no indication of phenotypic skewing at suboptimal levels. We also studied survival of T lymphocytes stripped of their TCRs. Unlike B cells, T cells lacking antigen receptors did not die precipitously; instead, populations decayed gradually, just as previously reported in the absence of MHC molecules.