The thymidine kinase (tk) gene of herpes simplex virus type 1 (HSV-1) was transduced into three glioma cell lines (T98, U251MG, T9) using a retrovirus vector. The supernatants of viral producer cell line PA317/LTRNL was used for infection and three transduced cell lines (T98/LTRNL, U251MG/LTRNL, T9/LTRNL) were established. The toxicities of the anti-herpetic drugs, acyclovir and gancyclovir, were evaluated in vitro. The cytotoxicities of acyclovir and gancyclovir to the HSV-1 tk gene-transduced cells increased 100-1000 fold compared to the non-transduced parental cell lines. The cytotoxic effect of gancyclovir was higher than that of acyclovir, requiring a concentration of less than 0.31 microM to obtain 50% inhibition. Deoxyribonucleic acid analysis of the gancyclovir-treated cells demonstrated fragmentation, suggesting that apoptosis is involved in the mechanism of cell death. The HSV-1 tk gene-transduced cells were co-cultured with parental cells and treated with gancyclovir. More than 95% of cells were killed in a mixture ratio of 1:1, suggesting that the "bystander effect" operated in this system. Selective transduction of HSV-1 tk gene into glioma cells using a retroviral vector and treatment with gancyclovir is a promising therapy for patients with malignant glioma.