Exposure of cells to hyperthermia is known to induce apoptosis, although the underlying mechanisms are only partially understood. Here, we examine the molecular requirements necessary for heat-induced apoptosis using genetically modified Jurkat T-lymphocytes. Cells stably overexpressing Bcl-2/Bcl-x(L) or stably depleted of Apaf-1 were completely resistant to heat-induced apoptosis, implicating the involvement of the mitochondria-mediated pathway. Pretreatment of wild-type cells with the cell-permeable biotinylated general caspase inhibitor b-VAD-fmk (biotin-Val-Ala-Asp(OMe)-CH(2)F) both inhibited heat-induced apoptosis and affinity-labeled activated initiator caspase-2, -8, and -9. Despite this finding, however, cells engineered to be deficient in caspase-8, caspase-2, or the caspase-2 adaptor protein RAIDD (receptor-interacting protein (RIP)-associated Ich-1/CED homologous protein with death domain) remained susceptible to heat-induced apoptosis. Additionally, b-VAD-fmk failed to label any activated initiator caspase in Apaf-1-deficient cells exposed to hyperthermia. Cells lacking Apaf-1 or the pro-apoptotic BH3-only protein Bid exhibited lower levels of heat-induced Bak activation, cytochrome c release, and loss of mitochondrial membrane potential, although cleavage of Bid to truncated Bid (tBid) occurred downstream of caspase-9 activation. Combined, the data suggest that caspase-9 is the critical initiator caspase activated during heat-induced apoptosis and that tBid may function to promote cytochrome c release during this process as part of a feed-forward amplification loop.