Interferon-γ (IFN-γ), as one of interferon family that regulates antiviral, antiproliferative, and immunomodulatory responses, has been implicated for the growth regulation of ovarian cancer cells. However, the molecular mechanisms are not yet fully defined. To analyze detailed mechanisms, the ovarian cancer cell lines (2774, PA-1, OVCAR-3, and SKOV-3) were treated with IFN-γ. The growth of 2774 was most effectively suppressed than that of other cells in both time-course and dose-dependent experiments. The order of sensitivity in other cells was PA-1 ≫ OVCAR-3 > SKOV-3 (not responded at all). The DNA fragmentation and DAPI staining assays suggested that the IFN-γ-mediated cytotoxicity could be triggered by apoptosis. The treatment induced IFN regulatory factor-1 (IRF-1) in two IFN-γ-sensitive cells (2774, PA-1), whereas IRF-1 was not induced in two IFN-γ-resistant cells (OVCAR-3, SKOV-3). The levels of p53 and p21WAF1 were not strikingly changed in all four cells. Interestingly, the expression of interleukin-converting enzyme (ICE, or caspase-1) was increased by the treatment in a kinetically consistent manner to the induction of IRF-1. However, CD95 (Fas/APO-1) was not changed. Apoptosis was greatly induced, when IRF-1 was transiently expressed in PA-1 without the treatment of IFN-γ. However, it was repressed when IRF-1 together with IRF-2, an antagonist of IRF-1, were coexpressed. In addition, the effect of IFN-γ was reduced in the 2774 and PA-1 cells stably expressing either IRF-1 antisense or IRF-2 sense, as shown by the cytotoxicity and FACS analysis. Furthermore, the IFN-γ-induced apoptosis was greatly reduced, when inhibitors of ICE were treated into PA-1 cells. Taken together, these results suggest that IRF-1 directly mediates the IFN-γ-induced apoptosis via the activation of caspase-1 gene expression in IFN-γ-sensitive ovarian cancer cells. J. Cell. Biochem. 85: 369–380, 2002. © 2002 Wiley-Liss, Inc.