We report virus-free transfer of a "suicide" gene into tumoral cells. The system can be used in vitro or in vivo to induce tumor cell death. A plasmid carrying the herpes simplex virus thymidine kinase (HSV-TK) gene with its 5'- and 3'-flanking regions was used both alone and in liposomes to transduce B16 cells. In vitro, a 5-day treatment with ganciclovir after transfection with the HSV-TK gene in liposomes induced a significant lysis of B16 melanoma cells as assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test. The efficacy of transfection was determined using liposomes harboring the beta-galactosidase reporter gene and was around 10%. Thus, the cytotoxicity observed resulted presumably from a large bystander effect. In vivo, direct transfer of the TK DNA into established B16 melanoma tumors in C57B6 mice followed by i.p. ganciclovir treatment induced a 50% reduction of tumor weight after 8 days and an increased necrosis. Despite the use of the nonspecific strong TK promoter, no necrosis was detected in normal tissues surrounding the tumor or elsewhere. Thus, this system of tumor transfection, which does not involve any viral vector, is safe and straightforward and seems to be suitable for testing in clinical trials.