Efficient antigen expression in vivo is an essential step in eliciting anti-tumour immunity by DNA vaccines. In this study we showed that inclusion of either Intron A of the human cytomegalovirus immediate early gene promoter or the SV40 promoter/enhancer in the vaccine vectors could increase the expression of an idiotypic (Id) Ig-FrC fusion construct in vitro, and incorporation of both components resulted in the highest expression level. Vaccination studies in mice showed that the levels of antigen expression in vitro correlated with the antibody responses in vivo. Importantly, higher antibody responses appeared to confer better protection against tumour challenge. Furthermore, higher level of antigen expression could also improve CD8⁺ T cell responses and provide better tumour protection when a DNA vaccine encoding that AH1 CTL epitope (pDOM-AH1) was tested. Together, the data strongly suggest that increasing antigen expression is an effective approach to augmenting the potency of DNA tumour vaccines. One strategy for enhancing the efficacy of tumour vaccines is to link to tumour antigen to a pathogen-derived antigen that can activate potent cognate CD⁺ T cell help. In this study, we explored the utilities of the B subunit of Escherichia coli heat-labile enterotoxin (EtxB). EtxB is not only a highly immunogenic molecule itself, but also can act as a potent adjuvant or carrier to increase immune responses to other antigens. Fusion of EtxB to a tumour antigen should provide linked CD4⁺ T cell help. Moreover, EtxB can directly exert potent immuno-modulatory activities on lymphocytes and antigen presenting cells. A further attraction is its potential to enhance activation of T cell responses by enhancing antigen via receptor-mediated mechanism.