A recently developed, adaptive constant-current electroporation technique was used to immunize mice with an intramuscular injection of plasmid coding for the extracellular and transmembrane domains of the product of the rat neu(664V-E) oncogene protein. In wild-type BALB/c mice, plasmid electroporation at lower current settings elicits higher antibody titers, a strong cytotoxic response and completely protects all mice vaccinated with 10, 25 and 50 microg of plasmid against a lethal challenge of rat neu+ carcinoma cells. BALB/c mice transgenic for the transforming rat neu(664V-E) (ErbB-2, Her-2/neu) oncogene (BALB-neuT(664V-E)) develop an invasive mammary gland carcinoma by 20 weeks of age. Remarkably, when transgenic BALB-neuT(664V-E) mice were vaccinated at a 10- week interval with 50 microg of plasmid with 0.2 A electroporation, mice remained tumor free for more than a year. A single administration of plasmid associated with electroporation was enough to markedly delay carcinogenesis progression in mice with multiple microscopic invasive carcinomas, and keep about 50% of mice tumor free at one year of age. Thus, vaccination using a clinically relevant dose of plasmid encoding the extracellular and transmembrane domains of the neu oncogene delivered by electroporation prevents long-term tumor formation. These improvements in the efficacy of this cancer vaccine regimen vastly increase its chances for clinical success.