To develop into committed T helper type 1 (Th1) cells, naive CD4(+) T cells not only need to acquire the capacity to produce interferon-gamma (IFN-gamma), but they also need to gain the ability to silence their interleukin-4 (IL-4) -producing potential. How Th1 cells silence their Th2 cytokine-producing potential is an important yet unresolved issue in Th1 immunity. We found that a lack of IL-4 stimulation was not sufficient to silence the IL-4-producing potential in activated CD4(+) T cells and that Th1-promoting factor was required. Although it has been shown that T-bet is a crucial factor in suppressing Il4 gene expression, it is unclear whether a continuous presence of T-bet is required to silence the Il4 gene in Th1 cells. To address this problem, we used an inducible form of T-bet - a T-bet-oestrogen receptor fusion molecule (T-bet-ER). We found that the activation of T-bet during primary or secondary culture was sufficient to silence IL-4-producing potential. On the other hand, the inactivation of T-bet after naïve CD4(+) T cells had differentiated into Th1 cells resulted in derepression of Il4 gene transcription. Additionally, we found that T-bet is required to maintain Ifng expression. Our data demonstrate that the continuous expression of T-bet is required for Th1 cells to silence their IL-4-producing potential.