Viral vectors used for cancer gene therapy have usually been either replication-incompetent vectors expressing a gene product that leads to the destruction of the tumor or replication-competent vectors that are inherently cytotoxic to the tumor cells. We have sought to combine the attributes of these different approaches using a defective herpes simplex virus (HSV) vector that consists of a defective particle, containing tandem repeats of the HSV thymidine kinase (TK) gene, and a replication-competent, non-neurovirulent HSV mutant as a helper virus. HSV-TK activity in defective vector-infected cells was significantly greater than that in helper virus-infected cells which contained a single copy of HSV-TK. Infection of cells with this defective vector renders them, as well as surrounding uninfected cells, sensitive to killing by ganciclovir. Ganciclovir treatment of C57BL/6 mice bearing TK-defective vector/helper virus-infected subcutaneous GL261 gliomas resulted in significantly decreased tumor size.