Copolymers designed for use as particulate carriers containing functionalizable amino acid subunits for coupling with targeting ligand are described. The copolymers are polyesters composed of .alpha.-hydroxy acid subunits such as D,L-lactide and .alpha.-amino acid subunits such as serine or in the preferred embodiment, terpolymers of D,L-lactide and glycolide and .alpha.-amino acid subunits such as serine. Stable vaccine preparations useful as delayed release formulations containing antigen(s) or antigen(s) and co-adjuvants encapsulated within or physically mixed with polymeric microparticles are described. The particulate carriers are useful for delivering agents to the immune system of a subject by mucosal or parenteral routes to produce immune responses, including antibody responses.