IFN-γ causes rapid induction of IL-12 responsiveness in TH1 cells primed in the absence of IFN-γ. TG CD4+ T cells were primed with peptide, APC, IL-2, anti-IL-12, anti-IFN-γ, and either IL-4 (TH2) or anti-IL-4 (TH1). (A) Cells were incubated for a total of 24 hr. IFN-γ (20 units/ml) was added at varying times as indicated. (B) Cells were cultured for 24 hr with IFN-γ (+) or anti-IFN-γ (−) and with IL-4 (+) or anti-IL-4 (−). Cells were then challenged with or without IL-12, and the capacity of the extracts to bind Fcγ RI GAS oligonucleotide sequence was determined as a measure of STAT-4 activity.