The purpose of this study was to investigate the effect of endotoxin presence in plasmid DNA preparations on the efficiency of transfection achieved in vivo with B16(F10) and Renca tumors and to determine transgene localization. Our data show that endotoxin markedly decreases the efficiency of transfection. Furthermore, the transgene transferred in vivo can be found in both neoplastic and normal (most likely myofibroblast) cells lying in proximity of the administration site.