Purpose. DNA-based vaccines encoding viral antigens have been shown to elicit immune responses in animal models. In this study, a plasmid DNA (pDNA) coexpressing the middle envelope protein of hepatitis B virus (HBV) and Interleukin-2 (IL-2) was incorporated into Poly (D, L-lactic-co-glycolic acid) (PLGA) microspheres and three different formulations were investigated for their potential as a vaccine delivery system. Methods. Emulsion solvent evaporation methods of water-in-oil-in-water (w/o/w) and oil-in-water (o/w) were used to generate three different formulations in which PLGA microspheres contained pDNA either encapsulated within or adsorbed onto the microspheres. Results. In vaccine formulation A2, prepared using the (w/o/w) method, pDNA was encapsulated within the microspheres. The other two formulations (B2 and B2a) were prepared using the (o/w) method and B2 contained pDNAs encapsulated within the microspheres while B2a contained pDNAs adsorbed onto the microspheres. pDNA loading efficiencies of A2, B2 and B2a were determined to be 15%, 25% and 45%, respectively. In vitro release of pDNAs from microspheres was evaluated for a 45-day period with no conformational changes and A2 displayed slower release than that of the B2 and B2a. When mice were immunized from anterior tibialis muscle using A2, B2 and B2a formulations containing 100 mu g pDNA, antibody responses were detected for 6 months in mice sera. Conclusions. Although all PLGA microsphere formulations containing pDNA elicited antibody responses by the end of the 6th month, the antibody titers were found to be higher with B2 and B2a formulations in comparison to A2 formulation and the naked pDNA in saline.