A decrease in the expression of Th1 cytokines has been associated with age-related decrease in cytotoxic T-lymphocyte (CTL) function. We utilized an E1-deleted adenovirus (Ad) vector to deliver the murine interleukin-12 (IL-12) gene in order to enhance the antivirus CTL response. Wild-type (WT) Ad was administered 3 days after AdIL-12 treatment, when IL-12 production was at its peak and the anti-Ad antibody response had not yet begun to develop. Before receiving AdIL-12 treatment, aged (18 month old) mice exhibited a 58% decrease in the number of virus-specific CTLs, and a 30% decrease in in vivo CTL activity as compared to young (2 month old) mice. After AdIL-12 treatment, aged mice displayed a greater increase in IL-12 expression and endogenous production of interferon-γ than observed in young mice. When infected with WT Ad, these AdIL-12-treated aged mice exhibited an increased in vivo CTL response and an in vitro proliferative response that was similar to those in young mice. The frequencies of occurrence of Db-E1Bp+ CD8+ T cells in the spleen, liver, and lung in aged mice were higher than the corresponding values in young mice. These results indicate that IL-12 treatment significantly promotes the virus-specific CTL response in aged mice and, more importantly, specifically targets the virally infected organs, such as the liver and lung, promoting enhanced CTL activity against the virus.