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To analyze the roles of interleukin-12 (IL-12) and the IL-12-dependent Th1 response in resistance to Cryptococcus neoformans , we have established a chronic infection model in wild-type mice and in mice with targeted disruptions of the genes for the IL-12p35 and IL-12p40 subunits (IL-12p35 −/− and IL-12p40 −/− mice, respectively) as well as in mice with a targeted disruption of the IL-4 gene. Long-term application of exogenous IL-12 prevented death of infected wild-type mice for the entire period of the experiment (up to 180 days) but did not resolve the infection. Infected IL-12p35 −/− and IL-12p40 −/− mice died significantly earlier than infected wild-type mice, whereas infection of IL-4-deficient mice led to prolonged survival. Interestingly, infected IL-12p40 −/− mice died earlier and developed higher organ burdens than IL-12p35 −/− mice, which, for the first time in an infection model, suggests a protective role of the IL-12p40 subunit independent of the IL-12 heterodimer. The fungal organ burdens of IL-4-deficient mice and IL-12-treated wild-type mice were significantly reduced compared to those of untreated wild-type mice and IL-12-deficient mice. Histopathological analysis revealed reduction of the number of granulomatous lesions following treatment with IL-12. Susceptibility of both IL-12p35 −/− and IL-12p40 −/− mice was associated with marginal production of gamma interferon and elevated levels of IL-4 from CD4 + T cells, which indicates Th2 polarization in the absence of IL-12, whereas wild-type mice developed a Th1 response. Taken together, our data emphasize the essential role of IL-12 for protective Th1 responses against C. neoformans .