Bax and Bid are proapoptotic proteins of the Bcl-2 family that regulate the release of apoptogenic factors from mitochondria. Although they localize constitutively in the cytoplasm, their apoptotic function is exerted at the mitochondrial outer membrane, and is related to their ability to form transbilayer pores. Here we report the poration activity of fragments from these two proteins, containing the first a-helix of a colicinlike hydrophobic hairpin (a-helix 5 of Bax and a-helix 6 of Bid). Both peptides readily bind to synthetic lipid vesicles, where they adopt predominantly a-helical structures and induce the release of entrapped calcein. In planar lipid membranes they form ion conducting channels, which in the case of the Bax-derived peptide are char- acterized by a two-stage pattern, a large conductivity and lipid-charge-dependent ionic selectivity. These features, together with the influence of intrinsic lipid curvature on the poration activity and the existence of two helical stretches of different orientations for the membrane-bound peptide, suggest that it forms mixed lipidic/peptidic pores of toroidal structure. In contrast, the assayed Bid fragment shows a markedly different behavior, characterized by the formation of discrete, steplike channels in planar lipid bilayers, as expected for a peptidic pore lined by a bundle of helices.