It is now well appreciated that the cytokines interleukin (IL)-12 and IL-4 are important for the generation of Th1 and Th2 cells, respectively. Only recently, however, have the molecular mechanisms by which these cytokines affect Th cell differentiation begun to be defined. Previous work from our laboratory has demonstrated that members of the signal transducer and activator of transcription (STAT) gene family are critical for the differentiation of Th cell subsets. In particular, Stat4-deficient mice show an impairment in the generation of Th1 cells, whereas Stat6-deficient animals do not generate Th2 cells. We have now generated Stat4-Stat6 double-deficient mice to determine whether STAT-independent pathways exist for the development of Th cell subsets. It is surprising that Th1 but not Th2 cells can be generated from double-deficient mice in vitro and these animals are able to mount an in vivo Th1 cell-mediated immune response. These results suggest a model of Th cell differentiation in which Stat4 and Stat6 have different roles in the development of Th cell subsets.