The aim of this study was to examine the contribution of IL-18 in host defense against infection caused by Cryptococcus neoformans in mice with defective IL-12 production. Experiments were conducted in mice with a targeted disruption of the gene for IL-12p40 subunit (IL-12p40−/− mice). In these mice, host resistance was impaired, as shown by increased number of organisms in both lungs and brains, compared with control mice. Serum IFN-γ was still detected in these mice at a considerable level (20–30% of that in control mice). The host resistance was moderately impaired in IL-12p40−/− mice compared with IFN-γ−/− mice. Neutralizing anti-IFN-γ mAb further increased the lung burdens of organisms. In addition, treatment with neutralizing anti-IL-18 Ab almost completely abrogated the production of IFN-γ and also impaired the host resistance. Host resistance in IL-12p40−/− IL-18−/− mice was more profoundly impaired than in IL-12p40−/− mice. Administration of IL-12 as well as IL-18 increased the serum levels of IFN-γ and significantly restored the reduced host resistance. Spleen cells obtained from infected IL-12p40−/− mice did not produce any IFN-γ upon restimulation with the same organisms, while those from infected and IL-12-treated mice produced IFN-γ. In contrast, IL-18 did not show such effect. Finally, depletion of NK cells by anti-asialo GM1 Ab mostly abrogated the residual production of IFN-γ in IL-12p40−/− mice. Our results indicate that IL-18 contributes to host resistance to cryptococcal infection through the induction of IFN-γ production by NK cells, but not through the development of Th1 cells, under the condition in which IL-12 synthesis is deficient.