Fatty acids are known promoters of apoptosis. In the present study, the direct role of fatty acids with regard to their ability to cause membrane permeabilization by Bax was explored. Addition of fatty acids to liposomes in the presence of cations greatly enhanced the permeabilizing activity of Bax, a pro-apoptotic Bcl-2 protein. This provides a putative mechanism for the role of fatty acids in apoptosis. It is not a result of detergent-like properties of fatty acids, since a different micelle-forming amphiphile, dilysocardiolipin, was strongly inhibitory. We also demonstrate that there is a synergistic effect on Bax-induced permeabilization between Ca2+ and Mg2+, both on the binding of Bax to liposomes as well as on the induction of the leakage of liposomal contents. Micromolar concentrations of Ca2+ added externally or submicromolar concentrations of free Ca2+ present in the medium were sufficient to promote Bax-induced permeabilization synergistically with externally added Mg2+. These results indicate that Bax can induce leakage from liposomes at ion concentrations resembling those found physiologically. The synergistic effects of Ca2+ and Mg2+ were observed with liposomes with different lipid compositions. Thus the action of Bax is strongly modulated by the presence of bivalent cations that can act synergistically, as well as by micelle-forming lipid components that can be either stimulatory or inhibitory.