Malignant gliomas are the most common malignant brain tumors and are almost always fatal. A thymidine kinase-negative mutant of herpes simplex virus-1 ( dl sptk) that is attenuated for neurovirulence was tested as a possible treatment for gliomas. In cell culture, dl sptk killed two long-term human glioma lines and three short-term human glioma cell populations. In nude mice with implanted subcutaneous and subrenal U87 human gliomas, intraneoplastic inoculation of dl sptk caused growth inhibition. In nude mice with intracranial U87 gliomas, intraneoplastic inoculation of dl sptk prolonged survival. Genetically engineered viruses such as dl sptk merit further evaluation as novel antineoplastic agents.