Different classes of anticancer drugs may trigger apoptosis by acting on different subcellular targets and by activating distinct signaling pathways. Here, we report that betulinic acid (BetA) is a prototype cytotoxic agent that triggers apoptosis by a direct effect on mitochondria. In isolated mitochondria, BetA directly induces loss of transmembrane potential independent of a benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone-inhibitable caspase. This is inhibited by bongkrekic acid, an agent that stabilizes the permeability transition pore complex. Mitochondria undergoing BetA-induced permeability transition mediate cleavage of caspase-8 (FLICE/MACH/Mch5) and caspase-3 (CPP32/Yama) in a cell-free system. Soluble factors such as cytochrome c or apoptosis-inducing factor released from BetA-treated mitochondria are sufficient for cleavage of caspases and nuclear fragmentation. Addition of cytochrome c to cytosolic extracts results in cleavage of caspase-3, but not of caspase-8. However, supernatants of mitochondria, which have undergone permeability transition, and partially purified apoptosis-inducing factor activate both caspase-8 and caspase-3 in cytosolic extracts and suffice to activate recombinant caspase-8. These findings show that induction of mitochondrial permeability transition alone is sufficient to trigger the full apoptosis program and that some cytotoxic drugs such as BetA may induce apoptosis via a direct effect on mitochondria.