Protection induced by vaccination depends on the capacity of the vaccine to elicit an appropriate immune response. In leishmaniasis, protection requires leishmanial-specific CD4+ T helper (T(H)) cells. Vaccination of BAL-B/c mice with leishmanial antigens and interleukin-12 (IL-12) promoted the development of leishmanial-specific CD4+ T(H)1 cells These mice were resistant to subsequent infection with Leishmania major. Thus, IL-12 is an effective adjuvant for the initiation of protective cell-mediated immunity against leishmaniasis and may be an important component in other vaccines that need tc induce cell-mediated immunity.