IL-4 was found to be the essential differentiation factor for Th2 cells and simultaneously to be a potent inhibitor of Th1 development that is induced by IFN-γ and IL-12. Furthermore, it was demonstrated that TGF-β can also inhibit Th1 development. In this work, we demonstrate that polyclonal activation of Mel-14highCD4+ T cells by immobilized anti-αβTCR mAb together with a mixture of IL-4 and TGF-β can lead to the development of both Th1 and Th2 cells, depending on the concentration of these cytokines. Additional experiments revealed that Th1 induction by a combination of IL-4 and TGF-β depends on the presence of endogenous IFN-γ, and that this alternative Th1 development is further enhanced by IL-12, but is not dependent on this cytokine. Moreover, naive OVA323–339-specific Th cells that were stimulated by APCs and OVA323–339 peptide differentiated toward Th1 cells after priming in the presence of IL-4 in combination with TGF-β. Hence, this finding confirmed the results obtained by polyclonal activation of naive CD4+ Th cells and implicates that this alternative Th1 development may also occur in vivo under the influence of TGF-β and IL-4 independently of the Th1-promoting effect of IL-12.