Hepatitis B virus (HBV) causes acute self-limiting and chronic infection in humans (24). A chronic HBV infection leads to a high risk for the development of liver cirrhosis and hepatocellular carcinoma (30, 54). The current strategy for preventing HBV infection is vaccination with hepatitis B surface antigen (HBsAg), which induces virus-neutralizing anti-HBsAg antibodies (28). Though HBsAg is a potent immunogen and induces protective immunity in the majority of vaccines, 5 to 10% of persons who receive the HBsAg vaccine failed to develop anti-HBsAg antibodies. In addition, HBV variants carrying mutations within the HBsAg can escape the neutralization of vaccine-induced anti-HBsAg and establish acute or chronic infection (3, 4, 6, 25, 29, 43). Therefore, a new vaccine strategy would be desirable to induce a multiple immune response consisting of HBV-specific T helper (Th), cytotoxic T cells (CTLs), and anti-HBsAg antibodies. HBV-specific Th and CTL responses play a pivotal role for the clearance of virus in a primary HBV infection and may control HBV persisting in unknown reservoirs in patients whose disease is resolved (1, 7, 16, 18, 26, 27, 35, 41, 42, 44, 45). The induction of HBV-specific humoral and cellular immune response by a single vaccine may overcome the nonresponsiveness of individuals to conventional HBsAg vaccines and control immune escape variants of HBV with mutations within HBsAg. DNA vaccination is a powerful method to induce antigen-specific humoral and cellular immune response (14, 56). DNA-induced immune response provides protective immunity to various viruses in animal models (2, 5, 13, 19, 22, 31, 34, 36, 51, 55, 57). Genetic vaccination to HBsAg, HBV core antigen (HBcAg), and HBV e antigen (HBeAg) was evaluated in different animal models. In mice, a single intramuscular injection of plasmids expressing HBsAg is sufficient to induce a long-lasting humoral response to HBsAg and CTL response (10, 12, 40, 50). A plasmid vaccination of chimpanzees led to the production of low anti-HBsAg antibody titers (11, 47). Recently, Triyatni et al. reported that vaccination of ducks with plasmid expressing duck hepatitis B virus (DHBV) surface antigens (DHBsAg) induced antibodies to DHBsAg (55). Anti-DHBsAg antibodies induced by DNA vaccination were able to neutralize virus in vitro. DHBV was removed more rapidly from the bloodstreams of vaccinated ducks after a challenge. Infection of hepatocytes by DHBV was limited or prevented in vaccinated ducks. Therefore, the genetic vaccination was effective to prime an anti-HBsAg antibody response in this model. The vaccination of mice with HBcAg or HBeAg was also effective for inducing specific CTL responses (33). The woodchuck (Marmota monax) model is useful to study immune response to hepadnavirus and to perform vaccination trials (8, 9, 23, 39, 48, 49, 52). Woodchuck hepatitis virus (WHV) causes acute self-limiting and chronic infection, like HBV in humans (53). The humoral immune responses to woodchuck hepatitis surface antigen (WHsAg) and core antigen (WHcAg) in acute and chronic WHV infection have the same features as those of HBV infection. Anti-WHcAg develops in woodchucks during the early phase of a primary WHV infection and persists lifelong. Anti-WHsAgs, like anti-HBsAgs, increase at the end of the viremic phase and may provide immunity to a secondary WHV infection. Recently, T-cell response to WHsAg and WHcAg in woodchucks during acute and chronic WHV infection was investigated by an in vitro assay to measure the antigen-specific proliferation of peripheral blood mononuclear cells (PBMCs) (8, 32, 38, 39). Multispecific Th response to WHcAg and WHsAg was present during acute WHV infection but absent in woodchucks with chronic WHV infection (39). Thus, the Th response to WHV in woodchucks closely resembles the HBV-specific Th response in humans (52). The woodchuck model is informative in the study of immune response induced by vaccines and virus challenge. Immunization of woodchucks with WHsAg-induced anti-WHsAg antibodies provided protection against a subsequent challenge with WHV (9). Interestingly, woodchucks immunized with WHcAg were protected against WHV challenge even though anti-WHcAg antibodies do not possess the ability to neutralize WHV (49, 52). Apparently, WHcAg induced a specific T-cell response which conferred protective immunity (39). We demonstrated that immunization with a peptide containing a T-cell epitope derived from WHcAg leads to the protection of woodchucks against WHV infection. These results emphasize the significance of T-cell response to the core antigen for control of hepadnavirus infection (16, 18). In the present study, we wanted to determine whether vaccination of woodchucks with plasmids expressing WHV proteins can induce a protective immune response to WHV. We vaccinated mice and woodchucks with plasmids expressing WHcAg and WHsAg and investigated the humoral and cellular immune response to WHcAg and WHsAg in woodchucks. The protective efficacy of plasmid vaccination was demonstrated in woodchucks in subsequent challenge experiments.