The infection of mice with Leishmania major parasite induces polarized Th1 and Th2 responses that cannot be significantly changed in vivo after 2 to 3 wk of infection by using either cytokines or anti-cytokine Abs. It is not clear, however, whether the T cell populations are irreversibly differentiated or whether the inability to modify the cytokine production reflects inefficiencies in the experimental treatments or complications of the infection itself. To study this further, we have cultured CD4+ T cells from L. major-infected mice with specific Ag, APC, and IL-2, in the presence or absence of different cytokines and/or anti-cytokine Abs. Th1 cells cultured for 1 wk in the presence of IL-4 produced very low levels of IFN-gamma but, instead, produced high levels of IL-4 and IL-10, suggesting that IL-4 was able to cause the conversion of a Th1 into a Th2 population. The Th2-like population generated in vitro was stable and retained its phenotype in vivo when transferred into L. major-infected C.B-17 scid mice. In contrast, the presence of IFN-gamma and IL-12 during the Th2 cell stimulation enhanced IFN-gamma production but was not sufficient to induce a complete conversion of a Th2 into a Th1-like population. Taken together, these data show that highly polarized murine Th populations can be modified and even converted to the opposite cytokine phenotype in vitro, suggesting possible therapeutic applications for cytokines.