Immunogenicity of tetanus toxoid (TT) encapsulated in biodegradable polymer microspheres composed of poly lactide (PLA) or poly lactide-co-glycolide (PLGA) polymers was evaluated in mice and guinea pigs for 1 year. Microsphere formulations made from polymers differing in molecular weight and composition elicited significantly higher IgG antibody levels than soluble TT in mice. The antibody levels elicited by microsphere formulations in mice and guinea pigs were similar to those elicited by a single injection of AlPO4 adsorbed TT. Immunogenicity was not consistently better with a particular polymer composition, molecular weight or microsphere size. However, animals primed with TT-containing microspheres showed significantly higher anamnestic response to a low dose booster 1 year after priming than those primed with AlPO4 adsorbed TT. Microspheres made from low molecular weight PLGA polymer showed a minimal local tissue reaction 1 year after injection. In contrast, aluminum adjuvant formed local granulomas which persisted for 1 year after injection. Microsphere formulations used in this study released a small fraction of antigenic TT during in vitro release studies due to denaturation of TT during encapsulation and hydration of microspheres. Nevertheless, strong priming of immune responses were seen. It remains to be demonstrated whether stabilization of TT would lead to more immunogenic microsphere formulations.