In gene therapy, the herpes simplex virus thymidine kinase (HSV-tk) gene is widely used as a suicide agent. Tumor cells expressing HSV-tk are sensitive to nucleoside analogs such as ganciclovir (GCV). An advantage of this system is the bystander killing effect whereby HSV-tk-positive cells exposed to GCV are lethal to surrounding HSV-tk-negative cells. We transfected the HSV-tk gene into a human cervical adenocarcinoma cell line, BU25TK-, and a human endometrial adenocarcinoma cell line, HHUA, by the Lipofectine method. The sensitivity of HSV-tk-positive cells to GCV and bystander killing effect on HSV-tk-negative cells were examined in vitro. HSV-tk-positive cells were sensitive to GCV at concentrations of 1 to 100 microg/ml in a dose- and time-dependent manner. The growth of HSV-tk-negative cells was inhibited when the population of cultured cells contained more than about 3% HSV-tk-positive cells. Moreover, for BU25TK- cells, HSV-tk-positive cells were injected into SCID mice subcutaneously and the effects of GCV therapy and bystander killing at a daily concentration of 25 mg/kg for 14 days were examined. HSV-tk-positive tumors transduced into SCID mice almost disappeared upon GCV treatment. Furthermore, tumor reduction was observed when mixtures of HSV-tk-negative cells containing more than 20% HSV-tk-positive cells were injected into SCID mice. In conclusion, the HSV-tk/GCV system might be applied to both cervical and endometrial adenocarcinoma.