IL-18 is a cytokine that is secreted from activated macrophages and induces IFNγ production. To investigate the in vivo role of IL-18, we generated IL-18–deficient mice. In Propionibacterium acnes (P. acnes)–primed IL-18–deficient mice, LPS-induced IFNγ production was markedly reduced, despite normal IL-12 induction. Natural killer cell activity was significantly impaired. Th1 cell response after injection of P. acnes or Mycobacterium bovis (bacillus Calmette-Guérin [BCG]) was significantly reduced. Similar results were observed in IL-12–deficient mice. Interestingly, Th1 response was induced after BCG infection in IL-12–deficient mice. We therefore generated mice lacking both IL-18 and IL-12. In these mice, NK activity and Th1 response were further impaired. This demonstrates the important role of both IL-18 and IL-12 in NK activity, as well as in in vivo Th1 response.