In response to various apoptotic stimuli, Bax, a pro-apoptotic member of the Bcl-2 family, is oligomerized and permeabilizes the mitochondrial outer membrane to apoptogenic factors, including cytochrome c. Bax oligomerization can also be induced by incubating isolated mitochondria containing endogenous Bax with recombinant tBid (caspase-8-cleaved Bid) in vitro. The mechanism by which Bax oligomerizes under these conditions is still unknown. To address this question, recombinant human full-length Bax was purified as a monomeric protein. Bax failed to oligomerize spontaneously in isolated mitochondria or in liposomes composed of either cardiolipin or lipids extracted from mitochondria. However, in the presence of tBid, the protein formed large complexes in mitochondrial membranes and induced the release of cytochrome c. tBid also induced Bax oligomerization in isolated mitochondrial outer membranes, but not in other membranes, such as plasma membranes or microsomes. Moreover, tBid-induced Bax oligomerization was inhibited when mitochondria were pretreated with protease K. The presence of the voltage-dependent anion channel was not required either for Bax oligomerization or for Bax-induced cytochrome c release. Finally, Bax oligomerization was reconstituted in proteoliposomes made from mitochondrial membrane proteins. These findings imply that tBid is necessary but not sufficient for Bax oligomerization; a mitochondrial protein is also required.