Escape of tumor cells from apoptotic-mediated stimuli results in tumor cell survival and resistance to cytotoxic mechanisms. Kaposi's sarcoma (KS) is the most common malignancy associated with AIDS, although its pathogenesis is not known. It is clinically important to determine whether AIDS-KS cells are resistant to apoptosis via the Fas system. Three isolates of AIDS-KS cells were studied. Although all KS cells express Fas on the cell surface, these cells were resistant to cytotoxic anti-Fas antibody (IgM, CH-11). Treatment of AIDS-KS cells with actinomycin D sensitized the tumor cells to anti-Fas cytotoxicity and apoptosis. Apoptosis was assessed by morphological changes and DNA fragmentation analysis. Three possible mechanisms related to AIDS-KS cells, resistance to anti-Fas cytotoxicity were examined. First, synthesis and secretion of soluble Fas by the tumor cells can neutralize antibody-induced cytotoxicity. However, none of the three types of KS cells expressed soluble Fas mRNA as determined by reverse transcription (RT)-PCR. Second, the expression of the proto-oncogene bcl-2 can protect cells from apoptotic signals. Analysis of bcl-2 mRNA by RT-PCR revealed that all three AIDS-KS cells express very low levels of bcl-2 mRNA. Third, the Fas-associated phosphatase-1 (FAP-1) is an antiapoptotic molecule reported to interact with Fas and can block transduction of the apoptotic signal. RT-PCR analysis revealed that all three types of AIDS-KS cells express high levels of FAP-1 mRNA, and treatment of KS cells with actinomycin D reduced the levels of FAP-1 mRNA significantly. These findings demonstrate that AIDS-KS cells are resistant to Fas-mediated apoptosis and suggest that FAP-1 may be involved in the acquisition of resistance of AIDS-KS to anti-Fas antibody-mediated apoptosis.