Plasmid encoded exogenous IL-12 delivered as a DNA vaccine adjuvant has been shown to improve vaccine-induced immunity. In particular, pIL-12 greatly improves antigen (Ag)-specific cytotoxic tlymphocyte (CTL) activity in immunized mice. The longevity of this response has not previously been studied in detail. We have studied the effect of co-immunization with pIL-12 on HIV gp160 and Influenza A Hemeagglutinnin-specific memory immune responses. Mice co-immunized with pIL-12 and plasmid encoded antigens maintained a greater memory response than those immunized with the plasmid antigen alone which could be measured at least 6 months after vaccination. Further, this translated to an improved outcome after challenge of long term rested mice that were previously immunized. The strength of the immune response as well as the number of Ag-specific T-cells is proportional to the number of Ag-specific cells primed by the vaccination regimen.