We previously showed that bacterial DNA contains immunostimulatory motifs consisting of unmethylated CpG dinucleotides flanked by two 5' purines and two 3' pyrimidines. These motifs rapidly trigger an innate immune response, characterized by the production of IL-6, IL-12, and IFN-gamma. Since DNA vaccines are constructed from plasmids of bacterial DNA, we examined whether CpG motifs present in these plasmids contributed to the immunogenicity of DNA vaccines. In vitro experiments showed that DNA plasmids induced production of the same cytokines stimulated by bacterial DNA, an effect eliminated by DNase treatment. In vivo experiments showed that the immunogenicity of a DNA vaccine was significantly reduced by methylating its CpG motifs and was significantly increased by coadministering exogenous CpG-containing DNA. These findings support the conclusion that CpG motifs in the plasmid backbone of DNA vaccines play an important role in the induction of Ag-specific immunity.