We demonstrated that IL-12 was induced during primary or secondary pulmonary adenoviral infection in wild-type (wt) mice. However, cellular responses were not compromised in the lungs of IL-12−/− mice. The level of IFN-γ in the lung was similar in wt and IL-12−/− mice during pulmonary viral infection. Upon Ag stimulation in vitro, lymphocytes from draining lymph nodes or spleen of infected IL-12−/− mice released large amounts of IFN-γ, but not IL-4, which were comparable to those released by wt lymphocytes. Furthermore, a predominantly IgG2a response to adenoviral infection was unimpaired in IL-12−/− mice. These significant anti-adenoviral Th1-type responses in IL-12−/− mice led to an efficient clearance of virus-infected cells in the lung. Whether IL-18 was involved in IL-12-independent anti-adenoviral immune responses was investigated. Abrogation of endogenous IL-18 by an Ab resulted in diminished IFN-γ release and lymphocytic infiltrate in the lung during adenoviral infection. Nevertheless, the development of lymphocytes of the Th1 phenotype was unimpaired in the absence of both IL-12 and IL-18. In contrast to their intact ability to mount Th1-type responses to viral infection, IL-12−/− mice suffered impaired Th1-type immune responses to pulmonary mycobacterial infection. Our findings suggest that IL-12, although induced, is not required for Th1-type responses to respiratory viral infection, in contrast to mycobacterial infection. IL-18 is required for the optimal release of IFN-γ in the lung during viral infection, but is not required for the generation of virus-reactive Th1-type lymphocytes. Th1 differentiation during respiratory adenoviral infection may involve molecules different from IL-12 or IL-18.