We have previously shown that mice lacking inducible NO synthase are markedly more susceptible to Leishmania major infection but developed a significantly enhanced Th1 cell response compared with wild‐type mice. Furthermore, at high concentrations, NO inhibited IL‐12 synthesis by activated macrophages, thereby indirectly suppressing the expansion of Th1 cells. We report here that at low concentrations, NO selectively enhanced the induction of Th1 cells and had no effect on Th2 cells. NO exerted this effect in synergy with IL‐12 during Th1 cell differentiation and had no effect on fully committed Th1 cells. NO appears to affect CD4⁺ T cells directly and not at the antigen‐presenting cells. These results therefore provide an additional pathway by which NO modulates the immune response and contributes to the homeostasis of the immune system.