AbstractIL-18 has been shown to play a critical role in the development of a Th1 response and immunity against intracellular pathogens. To determine the role of IL-18 in the development of protective immunity against Leishmania major, we have analyzed the course of cutaneous L. major in IL-18-deficient C57BL/6 mice (IL-18−/−) compared with similarly infected wild-type mice (IL-18+/+). After L. major infection, IL-18−/− mice may develop larger lesions during early phase of infection but eventually will resolve them as efficiently as IL-18+/+ mice. By 2 wk after infection, although Ag-stimulated lymph node cells from L. major-infected IL-18+/+ and IL-18−/− mice produced similar levels of IFN-γ, those from IL-18−/− mice produced significantly more IL-12 and IL-4. By 10 wk after infection, both IL-18+/+ and IL-18−/− mice had resolved L. major infection. At this time, lymph node cells from both IL-18+/+ and IL-18−/− mice produced IL-12 and IFN-γ but no IL-4. Furthermore, administration of anti-IFN-γ Abs to IL-18−/− mice rendered them susceptible to L. major. These results indicate that despite the role IL-18 may play in early control of cutaneous L. major lesion growth, this cytokine is not critical for development of protective Th1 response and resolution of L. major infection.