To further understand the interaction among GATA-3, Stat4, and T-bet in helper T cell development, we first showed that retroviral expression of GATA-3 in developing Th1 cells suppresses Th1 development through downregulation of Stat4 rather through downregulation of the IL-12Rβ2 chain. Correspondingly, Stat4 levels are greatly suppressed during physiological Th2 development. Then, using cells doubly infected with GFP- and YFP-expressing retroviruses, we showed that retroviral GATA-3 expression in developing Th1 cells does not block Th1 development in cells coexpressing Stat4 but does so in cells coexpressing T-bet. Finally, we showed that retroviral Stat4 expression could facilitate Th2→Th1 conversion in cells bearing an IL-12Rβ2 transgene, even in cells lacking T-bet. These findings reassert that Stat4 signaling is a central element of Th1/Th2 development.