A DNA vaccine for inducing a tumor immune response was investigated using a well-characterized murine model tumor antigen. We demonstrated that in vivo electroporation augmented the induction of IFNgamma enzyme-linked immunospot (ELISPOT) and cytotoxic T lymphocyte (CTL) generation against pRL1a peptide in BALB/c spleen cells upon immunization with RLakt plasmid. Immunization without in vivo electroporation resulted in only a marginal induction of IFNgamma ELISPOT and CTL generation. Furthermore, co-injection of GM-CSF and RLakt plasmids significantly enhanced the induction of IFNgamma ELISPOT and CTL generation compared to the injection of RLakt plasmid alone. Inhibition of RL male 1 tumor growth was observed by injecting BALB/c mice with GM-CSF and RLakt plasmids using in vivo electroporation, although no effect was observed against an established tumor using the same treatment. No growth inhibition was observed without in vivo electroporation. Immunization with either RLakt plasmid alone, or GM-CSF and pCIneo control plasmids using in vivo electroporation did not inhibit RL male 1 tumor growth.