In vivo electroporation dramatically enhances plasmid vaccine efficacy. This enhancement can be attributed to increased plasmid delivery and, possibly, to some undefined adjuvant properties. Previous reports have demonstrated CD8(+) T cell priming by plasmid vaccines is strongly dependent upon CD4(+) T cell help. Indeed, the efficacy of a plasmid vaccine expressing Escherichia coli beta-galactosidase was severely attenuated in MHC class II-deficient (C2D) mice. To determine whether electroporation could compensate for the absence of CD4(+) T cell help, C2D mice were immunized by a single administration of plasmid in combination with electroporation using two conditions which differed only by the duration of the pulse (20 or 50 msec). Both conditions elicited robust cellular and humoral responses in wild-type mice, as measured by IFN-gamma ELISPOT, anti-beta-galactosidase ELISA, and protection from virus challenge. In C2D mice, the cellular response produced by the vaccine combined with the 50-msec pulse, as measured by ELISPOT, was identical to the response in wild-type mice. The 20-msec pulse elicited a milder response that was approximately one-fifth that of the response elicited by the 50-msec pulse. By contrast, the 20-msec conditions provided comparable protection in both wild-type and C2D recipients whereas the protection elicited by the 50-msec conditions in C2D mice was weaker than in wild-type mice. Further investigation is required to understand the discordance between the ELISPOT results and outcome of virus challenge in the C2D mice. Nonetheless, using this technique to prime CD8(+) T cells using plasmid vaccines may prove extremely useful when immunizing hosts with limiting CD4(+) T cell function, such as AIDS patients.