A fusogenic liposome composition for delivering a liposome-entrapped compound into the cytoplasm of a target cell is described. The liposomes have an outer surface coating of chemically releasable hydrophilic polymer chains which shield hydrophobic polymers on the liposome outer surface. Release of the hydrophilic polymer chains exposes the hydrophobic polymers for interaction with outer cell membranes of the target cells to promote fusion of the liposome with the target cells. Also disclosed is a polymer-lipid conjugate for use in promoting fusion between target membranes. The conjugate is composed of a first segment composed of a hydrophilic polymer and a second hydrophobic polymer segment. The second segment is joined to the first segment by a bond effective to release the first segment in response to an existing or an induced physiologic condition. Attached to the second segment is a vesicle-forming lipid member.