In this study we demonstrated that CD4 T cells from STAT4/mice exhibit reduced IL-12R expression and poor IL-12R signaling function. This raised the question of whether activated STAT4 participates in Th1 cell development mainly through its effects on IL-12 signaling. In a first approach to this question we determined the capacity of CD4 T cells from STAT4 /bearing an IL-12R 2 chain transgene (and thus capable of normal IL-12R expression and signaling) to undergo Th1 differentiation when stimulated by Con A and APCs. We found that such cells were still unable to exhibit IL-12-mediated IFN-production. In a second approach to this question, we created Th2 cell lines (D10 cells) transfected with STAT4-expressing plasmids with varioustyrosine3phenylalanine mutations and CD4 T cell lines from IL-12 2/mice infected with retroviruses expressing similarly STAT4 mutations that nevertheless express surface IL-12R 2 chains. We then showed that constructs that were unable to support STAT4 tyrosine phosphorylation (in D10 cells) as a result of mutation were also incapable of supporting IL-12-induced IFN-production (in IL-12R 2/cells). Thus, by two complementary approaches we demonstrated that activated STAT4 has an essential downstream role in Th1 cell differentiation that is independent of its role in the support of IL-12R 2 chain signaling. This implies that STAT4 is an essential element in the early events of Th1 differentiation.