There is increasing evidence that IL-18 is a key pro-inflammatory cytokine and an important mediator of Th1 immune response. The main source of IL-18 is macrophage-like cells. In the present study we have investigated IL-18 protein expression in primary human macrophages in response to influenza A and Sendai virus infections. Macrophages constitutively expressed proIL-18 but produced biologically active IL-18 only after virus infection. The IL-18 release was due to virus infection-induced proteolytic processing of 24-kDa proIL-18 into its mature 18-kDa form. ProIL-18 processing required active caspase-1 enzyme and the release of mature IL-18 was blocked with a caspase-1-specific inhibitor. Caspase-3 inhibitor also reduced IL-18 production in response to virus infection. Inactive proforms of caspase-1 and caspase-3 were basally expressed in macrophages, and virus infection induced the cleavage of procaspases into their mature forms. Besides increasing the expression of caspase proteins, virus infection enhanced caspase mRNA expression in macrophages. The enhancement of caspase gene expression was abrogated by anti-IFN-alpha antibody. Furthermore, IFN-alpha and IFN-gamma could induce caspase gene expression. These results imply that interferons are involved in virus-induced caspase activation that leads to proIL-18 processing and subsequent release of mature IL-18.