The use of live attenuated invasive bacteria as a carrier for DNA-based vaccines has been reported recently. In this study, we used a Shigella flexneri serotype 2a rfbF mutant for immunization of a DNA vaccine coding for HIV-1 SF2 Gag. The recombinant bacterial vector delivered gag DNA to mammalian cells in vitro resulting in Gag protein expression, and was found to have a low level of pathogenicity among a number of Shigella cell spread defective mutants tested. Intranasal immunization of mice with live recombinant bacterial cells induced a gag-specific cellular immune response similar to that seen with i.m. injection of naked DNA. Importantly, a strong boosting effect was observed in mice primed with DNA, suggesting utility of bacterial vectors in prime-boost vaccination regimens.