A liposome composition is provided which contains a liposome having: (i) an outermost lipid bilayer containing, in addition to a neutral, bilayer preferring lipid, a fusion-promoting effective amount of an ionizable lipid having a protonatable, cationic headgroup and an unsaturated acyl chain; and (ii) a compartment adjacent to the outermost lipid bilayer which contains an aqueous solution having a first pH. External to the liposome in the composition is an aqueous solution having a second pH. The first pH is less than the pK.sub.a of the ionizable lipid in the outermost lipid bilayer and the second pH is greater than the pK.sub.a of the ionizable lipid in the outermost lipid bilayer, such that there is a pH gradient across the outermost lipid bilayer and the ionizable lipid is accumulated in the inner monolayer of the outermost lipid bilayer in response to the gradient. The liposome can be fused in a controlled manner to another lipid bilayer, for example, the plasma membrane of a mammalian cell, by degrading the pH gradient when fusion is to occur, such that the ionizable lipid is neutral, evenly distributed in the outermost lipid bilayer and, fusogenic. Controlled fusion can be used to control the delivery of a biologically active agent entrapped in the liposome into a cell.