Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is one of the latest members of the TNF superfamily known to induce apoptosis in a wide variety of tumor cells. Some cell types, however, are quite resistant to TRAIL. We investigated the effect of ectopic expression of Bcl-2 and Bcl-xL on TRAIL-induced apoptosis in human acute myelogenous leukemia HL-60 cells. We found that HL-60 cells, which express TRAIL receptors (also called death receptor, DR) DR4, DR5, and Dc (decoy) R2, are highly sensitive to TRAIL-induced cytotoxicity. Greater than 90% killing occurred within 24 h of TRAIL treatment. The expression of Bcl-2 and Bcl-xL, however, completely abolished the TRAIL-induced cytotoxic effects. Treatment of HL-60 cells with TRAIL induced caspase-8 activation within 2-4 h, but no activation could be seen in Bcl-2-expressing or Bcl-xL-expressing cells. TRAIL also induced cleavage of BID, which was also abolished by Bcl-2 and Bcl-xL. Similarly, TRAIL activated caspase-3 and caspase-7 in control cells but not in cells expressing Bcl-2 or Bcl-xL. Cleavage of the caspase-3 substrate poly(ADP-ribose) polymerase (PARP), was abrogated by ectopic expression of Bcl-2 and Bcl-xL. Inhibition of caspases by the pan-caspase inhibitor, benzyloxycarbonyl-valine-alanine-aspartate-fluoromethylketone (zVAD-fmk) abolished the TRAIL-induced apoptosis. Overall, these results indicate that TRAIL-induced apoptosis involves activation of caspase-8, caspase-7, caspase-3, and BID cleavage, and Bcl-2 and Bcl-xL prevents TRAIL-induced apoptosis by abrogating caspase activation and BID cleavage.