A method of releasing an agent for example, a chemotherapeutic, under predetermined conditions by protecting the agent within a lipid structure such as a liposome, causing lipase activity to be constituted by combining two or more components, e.g., recombinant N- or C-terminal Clostridium perfringens alpha-toxin fragments, one of these components being conjugated to a targeting molecule e.g., an antibody which binds to a target such as a tumor antigen. The lipid structure is then exposed to the constituted lipase activity such as to release the agent. Also disclosed are materials and kits for use in the method.