The differentiation of CD4(+) T cells into T helper type 1 (Th1) cells is driven by interleukin (IL)-12 through the IL-12 receptor beta2 (IL-12Rbeta2) chain, whereas differentiation into Th2 cells is driven by IL-4, which downregulates IL-12Rbeta2 chain. We reexamined such differentiation using IL-12Rbeta2 chain transgenic mice. We found that CD4(+) T cells from such mice were able to differentiate into Th2 cells when primed with IL-4 or IL-4 plus IL-12. In the latter case, the presence of IL-4 suppressed interferon (IFN)-gamma production 10-100-fold compared with cells cultured in IL-12 alone. Finally, in studies of the ability of IL-12 to convert Th2 cells bearing a competent IL-12R to the Th1 cells, we showed that: (a) T cells bearing the IL-12Rbeta2 chain transgene and primed under Th2 conditions could not be converted to Th1 cells by repeated restimulation under Th1 conditions; and (b) established Th2 clones transfected with the IL-12Rbeta2 chain construct continued to produce IL-4 when cultured with IL-12. These studies show that IL-4-driven Th2 differentiation can occur in the presence of persistent IL-12 signaling and that IL-4 inhibits IFN-gamma production under these circumstances. They also show that established Th2 cells cannot be converted to Th1 cells via IL-12 signaling.