The antiviral drug acyclovir, an analogue of purine, was found to selectively enhance the radiosensitivity of rodent tumor cells which were transduced with the herpes simplex virus thymidine kinase gene (HSV-tk). 9L rat glioma cells transduced with HSV-tk and treated with acyclovir (20 micrograms/ml) for 24 hr before or after irradiation were highly sensitive to radiation, as compared with non-transduced glioma cells. When 9L cells transduced with HSV-tk gene were exposed to acyclovir and radiation, the sensitizer enhancement ratio (SER) was 1.6. In vivo, a significant increase in the median survival time of rats with 9L-tk tumors was observed when acyclovir was administered before and after single-dose irradiation, relative to the survival time of similar rats receiving radiation alone. The results show that an antiviral agent can selectively enhance cell killing by radiation in cells transduced with the HSV-tk, and suggest that the addition of HSV-tk gene therapy to standard radiation therapy will improve the effectiveness of treatment for brain tumors.