The WHO has listed the development of single dose vaccines as one of the "grand challenges" for human health. Traditional vaccination regimes require 3 to 4 doses of an antigen to induce lasting immunity. This study focused on the development of a single dose vaccine based on biodegradable polyanhydrides. Tetanus toxoid (TT) was used as a model antigen. The microspheres evaluated were copolymers of sebacic acid and l,6-bis(p-carboxyphenoxy)hexane. By changing copolymer ratios, release kinetics of the antigen were modulated to obtain a sustained release, mimicking multiple doses of traditional vaccine. Mice (C3H/HeOuJ) were immunized once with different vaccine formulations of polyanhydride microspheres containing TT. Serum antibody titers to TT were evaluated over the course of 28 weeks. Following immunization with the TT-loaded microspheres, resultant antibody titers and avidity were comparable in magnitude and duration to that obtained in mice receiving an equivalent single dose of TT (3-10 [Mu]g). The results indicate that the efficacy of the immunization regimen and modulation of the immune response (i.e., Thl shift) improved by the addition of a small amount of soluble (i.e., non-encapsulated) TT (0.5 [Mu]g) along with the TT-loaded microspheres. Incorporation of 0.5 [Mu]g of TT along with TT-loaded microspheres in the vaccine induced a shift in the antibody isotype as demonstrated by decreased production of IgGl. Evidence based on antibody and recall proliferation responses show that polyanhydride microspheres can be used as an effective single dose vaccine delivery system.