Macromolecules, being unable to penetrate cell membranes readily, enter the cell via endocytosis. The contents of the endosome are eventually routed to enzyme-rich lysosomes and in all likelihood degraded. If pharmacological activity of a therapeutic macromolecule is realized only when it enters the cytosol, then a mechanism must be built within a delivery system by which these agents escape from the endosome into the cytosol. The endosomal pH drops to as low as 5.0 as the endosome matures. This provides a unique opportunity in devising such a strategy for cytosolic delivery. We have designed a series of acid-sensitive lipids that can be readily incorporated into liposomal drug carrier systems. Upon entering the cell and encountering the acidic environment of the endosome, the acid sensitive lipid breaks down to release potent detergents. Critical hypotheses being tested at this point are:(1) presence of these detergents initiates liposome-to-micelle transition that triggers catastrophic release of entrapped macromolecules within endosomes and (2) micelles then induce permeabilization of endosomal membrane to promote cytosolic transfer of macromolecules. However, it is impossible to test these hypotheses without incorporating a critical concentration of detergent into liposomal bilayers. This concentration, above which vesicle-to-micelle transition can take place is unattainably high simply because at high detergent concentrations, liposomes cannot be made. In the present study, we propose to synthesize "bis-detergents", inert double-tailed lipids that can be embedded at high concentrations in liposomal bilayers and break down only at low pH into "membrane-active" detergents. When realized, the technology developed from this study will have profound implication not only in basic biomedical research entailing cytosolic delivery of macromolecules in vitro but also in a variety of therapeutic applications in vivo, ranging from delivery of antigenic proteins for induction of cell-mediated immunity to nucleic acids for gene regulation.