Treatment of drug-sensitive or resistant tumor cells with combination of anti-Fas antibody and drugs (e.g., CDDP) results in augmented cytotoxicity and synergy. This study examined a possible underlying mechanism of synergy achieved by anti-Fas and CDDP. Three human ovarian tumor cell lines were selected for the studies, namely, the CDDP-sensitive A2780 and CDDP-resistant variants AD10 and C30 tumor cells. All three lines express Fas but are resistant to cytotoxicity by anti-Fas antibody. Treatment of tumor cells with monoclonal mouse antihuman Fas (IgM) antibody and CDDP resulted in significant augmentation of cytotoxicity and synergy in all three lines. The magnitude of synergy was a function of the concentrations of both the anti-Fas antibody and the CDDP used. Pretreatment of tumor cells first with CDDP, but not with anti-Fas and then treatment with anti-Fas, resulted in synergy, suggesting that CDDP sensitizes the cells to anti-Fas-mediated cytotoxicity. This was corroborated by inhibiting synergy by the addition of neutralizing anti-Fas (IgG) antibody. Sensitization of tumor cells by CDDP resulted in upregulation of surface Fas expression which was dependent on de novo protein synthesis. Findings similar to those obtained in cytotoxicity were also obtained in apoptosis as determined by DNA hypoploidy and DNA fragmentation. The effect of CDDP-mediated sensitization to anti-Fas and cytotoxicity was compared to CDDP-mediated toxicity. The addition of the antioxidant butylated hydroxyanisole (BHA) inhibited CDDP-mediated cytotoxicity in both AD10 and C30 but not in A2780 ovarian tumor cells. However, BHA did not inhibit upregulation of Fas expression by CDDP in all three lines and further BHA did not inhibit the synergy achieved with combination of subtoxic concentrations of CDDP and anti-Fas (IgM) antibody. These findings revealed that CDDP exerts its cytotoxic effect and its sensitization to Fas cytotoxicity by different mechanisms. Since cytotoxic T lymphocytes (CTL) express Fas ligand and kill Fas+ target cells, a significant potentiation of tumor cell killing will be achieved following sensitization of tumor cells to Fas signaling by subtoxic concentrations of CDDP. These findings suggest a new approach for augmenting CTL-mediated immune interventions in the therapy of resistant ovarian tumor cells.