A replication-defective adenovirus-LacZ recombinant virus (AdLacZ) was injected intravenously into IRF-1-/- mice and wild-type mice to characterize the contribution of IRF-1 to the immune-mediated clearance of Ad vector. Compared with wild-type mice, IRF-1-/- mice expressed higher levels of the LacZ gene product in the liver. After infusion of the AdLacZ, the expression of IRF-1 mRNA was upregulated in the liver of wild-type mice, but not in IRF-1-/- mice. Both spleen and liver mononuclear cells from IRF-1-/- mice initially exhibited a markedly lower number of NK, NK-T and CD8 T cells. At day 7 after the administration of AdLacZ, there was a significantly increased population of NK, NK-T and CD8 T cells in both spleen and liver, and also CD11b+ cells in liver of IRF-1-/- mice, compared with the increased in wild-type mice. As IRF-1 is an important signal for production of IFN-γ by CD8 T and NK cells as well as production of IL-12 by CD11b+ cells, we determined whether there were lower levels of these cytokines in IRF-1-/- mice after Ad challenge. Surprisingly, there were lower levels of IL-12, but higher levels of IFN-γ and IL-18 in IRF-1-/- compared with wild-type mice at day 7 after administration with AdLacZ. These results indicate that delayed clearance of Ad is associated with partial correction of defects of the NK, NK-T and CD8 T cells and increased production of IFN-γ and IL-18 in IRF-1-/- mice.