The transcription factor GATA-3 is expressed in T helper 2 (TH2) but not TH1 cells and plays a critical role in TH2 differentiation and allergic airway inflammation in vivo. Mice that lack the p50 subunit of nuclear factor B (NF-B) are unable to mount airway eosinophilic inflammation. We show here that this is not due to defects in TH2 cell recruitment but due to the inability of the p50-/- mice to produce interleukin 4 (IL-4), IL-5 and IL-13: cytokines that play distinct roles in asthma pathogenesis. CD4+ T cells from p50-/- mice failed to induce Gata3 expression under TH2-differentiating conditions but showed unimpaired T-bet expression and interferon (IFN-) production under TH1-differentiating conditions. Inhibition of NF-B activity prevented GATA-3 expression and TH2 cytokine production in developing, but not committed, TH2 cells. Our studies provide a molecular basis for the need for both T cell receptor and cytokine signaling for GATA-3 expression and, in turn, TH2 differentiation.