Provided herein is a novel composition useful in assembling carrier vesicles for delivery of a biologically active agent to an animal. The composition comprises an amphiphilic triblock copolymer comprised entirely of biocompatible, biodegradable, and/or enzymatically degradable polymers. The composition is characterized by the ability to self assemble into an aqueous vesicle, thereby encapsulating an agent for delivery to the animal. Also provided is a method for making a composition for delivery of an agent and a method for administering the agent to an animal.