Electroporation was applied to enhance gene transfer into subcutaneous MC2 murine breast tumors. Cultured MC2 cells were also transfected by electroporation or by cationic liposomes in the presence of serum using pSV-luc plasmids. Electroporation parameters and liposome formulation were optimized to achieve the highest relative levels of transfection. An electric field threshold for successful electrotransfection in cultured cells appeared around 800-900 V/cm. The liposomes used contained the cationic lipid dioleoyl-3-trimethylammonium propane (DOTAP). Multilamellar vesicles (MLV) had a 10-fold advantage over small unilamellar vesicles (SUV) in cell culture transfection. For in vivo gene delivery, the plasmids were injected either alone, or in complex with MLV or SUV DOTAP liposomes. A series of six electric