VDAC is a channel-forming protein, located in the outer mitochondrial membrane, whose properties are consistent with the known permeability behavior of that membrane. When extracted with Triton X-100, VDAC exists as a 110,000 molecular weight glycoprotein complex which is a prepackaged channel. When inserted into liposomes, the permeability increase to non-electrolytes is consistent with a pore radius of 20. In a planar lipid bilayer, VDAC is anion selective and voltage gatable. This suggests that the permeability pathway in the outer mitochondrial membrane might be under physiological control. Many of VDAC's properties are qualitatively very similar, although quantitatively different, to those of porin, the channel responsible for the permeability of the outer membrane of at least some gram negative bacteria.