Proteins of the B-cell lymphoma protein 2 (Bcl2) family are key regulators of the apoptotic cascade, controlling the release of apoptotic factors from the mitochondrial intermembrane space. A helical hairpin found in the core of water-soluble folds of these proteins has been reported to be the pore- forming domain. Here we show that peptides including any of the two a-helix fragments of the hairpin of Bcl2 associated protein X (Bax) can independently induce release of large labelled dextrans from synthetic lipid vesicles. The permeability promoted by these peptides is influenced by intrinsic monolayer curvature and accompanied by fast transbilayer redis- tribution of lipids, supporting a toroidal pore mechanism as in the case of the full-length protein. However, compared with the pores made by com- plete Bax, the pores made by the Bax peptides are smaller and do not need the concerted action of tBid. These data indicate that the sequences of both fragments of the hairpin contain the principal physicochemical require- ments for pore formation, showing a parallel between the permeabilization mechanism of a complex regulated protein system, such as Bax, and the much simpler pore-forming antibiotic peptides.