Reduced antigen expression after DNA vaccination could be a factor that limits its therapeutic effects in antigen-expressing tumor-bearing hosts. To examine whether the presence of an antigen-positive tumor results in the destroy of antigen-expressing cells, pCMV-OVA plasmid expressing ovalbumin (OVA), a model antigen, was intradermaly injected to tumor-free mice and those bearing an EG7-OVA tumor, which stably expresses OVA. For a quantitative evaluation of the elimination of antigen-expressing cells, a plasmid independently expressing firefly luciferase and OVA (pCMV-Luc/OVA) was injected to mice that had been untreated or administered with pCMV-OVA, and luciferase activity was measured one week after the administration. In both tumor-free and tumor-bearing mice, a pre-injection of pCMV-OVA reduced the luciferase expression from pCMV-OVA/Luc compared with a pre-injection of an empty plasmid, suggesting that an immune response to antigen-expressing cells is induced. In tumor-bearing mice, OVA-specific humoral and cellular immune responses were detected even before the vaccination, but such responses had no significant effects on the luciferase expression from pCMV-OVA/Luc. These results indicate that DNA vaccination can induce antigen-specific immune response even in tumor-bearing mice, although it is not strong enough to reject the tumor burden.