Host resistance to the intracellular protozoan Leishmania major is highly dependent on IL-12 production by APCs. Genetically resistant C57BL/6 mice develop IL-12-mediated Th1 immune response dominated by IFN-gamma and exhibit only small cutaneous lesions that resolve spontaneously. In contrast, because of several genetic differences, BALB/c mice develop an IL-4-mediated Th2 immune response and a chronic mutilating disease. Myeloid differentiation marker 88 (MyD88) is an adaptator protein that links the IL-1/Toll-like receptor family to IL-1R-associated protein kinase. Toll-like receptors recognize pathogen associated molecular patterns and are crucially implicated in the induction of IL-12 secretion by APC. The role of MyD88 protein in the development of protective immune response against parasites is largely unknown. Following inoculation of L. major, MyD88(-/-) C57BL/6 mice presented large footpad lesions containing numerous infected cells and frequent mutilations. In response to soluble Leishmania Ag, cells from lesion-draining lymph node showed a typical Th2 profile, similar to infected BALB/c mice. IL-12p40 plasma level collapses in infected MyD88(-/-) mice compared with infected wild-type C57BL/6 mice. Importantly, administration of exogenous IL-12 rescues L. major-infected MyD88(-/-) mice, demonstrating that the susceptibility of these mice is a direct consequence of IL-12 deficiency. In conclusion, MyD88-dependent pathways appear essential for the development of the protective IL-12-mediated Th1 response against the Leishmania major parasite. In absence of MyD88 protein, infected mice develop a nonprotective Th2 response.