The Ets transcription factor PU. 1 is a master regulator for the development of multiple lineages during hematopoiesis. The expression pattern of PU. 1 is dynamically regulated during early T lineage development in the thymus. We previously revealed that PU. 1 delineates heterogeneity of effector Th2 populations. In this study, we further define the function of PU. 1 on the Th2 phenotype using mice that specifically lack PU. 1 in T cells using an lck-Cre transgene with a conditional Sfpi1 allele (Sfpi1 lck−/−). Although deletion of PU. 1 by the lck-Cre transgene does not affect T cell development, Sfpi1 lck−/− T cells have a lower activation threshold than wild-type T cells. When TCR engagement is limiting, Sfpi1 lck−/− T cells cultured in Th2 polarizing conditions secrete higher levels of Th2 cytokines and have greater cytokine homogeneity than wild-type cells. We show that PU. 1 modulates the levels of TCR expression in CD4+ T cells by regulating the DNA-binding activity of GATA-3 and limiting GATA-3 regulation of TCR gene expression. GATA-3-dependent regulation of TCR expression is also observed in Th1 and Th2 cells. In CD4+ T cells, PU. 1 expression segregates into subpopulations of cells that have lower levels of surface TCR, suggesting that PU. 1 contributes to the heterogeneity of TCR expression. Thus, we have identified a mechanism whereby increased GATA-3 function in the absence of the antagonizing activity of PU. 1 leads to increased TCR expression, a reduced activation threshold, and increased homogeneity in Th2 populations.