Entry of herpes simplex virus (HSV) into tumor cells results in viral gene expression followed by cellular lysis. Attenuated HSVs selectively destroy tumors with sparing of surrounding normal tissue. HSV encodes a thymidine kinase (TK) that converts ganciclovir to a toxic metabolite. This metabolite may be transferred between cells and lead to the death of neighboring uninfected cells, termed bystanders. We sought to determine if HSV-mediated oncolysis is enhanced by ganciclovir treatment. In addition, we examined bystander killing in cocultures of TK transfectants and parental cells. hrR3, an attenuated HSV, expresses TK. The 50% lethal dose of hrR3 for a rat gliosarcoma (9L) and three human colorectal carcinomas (HT29, KM12C6, and KM12L4) was determined. Cells were infected with a 50% lethal dose of hrR3, followed by treatment with ganciclovir, and then cell survival was quantitated. In separate experiments 9L and HT29 cells were transfected with TK. Parental cells and TK transfectants were cocultured in various ratios, in the presence of ganciclovir, and cell survival was quantitated. hrR3-mediated oncolysis was enhanced by ganciclovir in the gliosarcoma but not in the three colorectal carcinomas. Cocultures of both 9L and HT29 parental cells with their corresponding TK transfectants demonstrated bystander killing. The mortality of 9L cocultures was 54% greater than that predicted for exclusive killing of transfectants. HT29 mortality was 8% greater than predicted. The ability of ganciclovir to augment hrR3-mediated oncolysis varies significantly between tumor cells lines. The extent of ganciclovir-mediated killing of neighboring nontransduced parental cells similarly varies. Consideration should be given to these factors in the design of gene therapy strategies using HSV vectors as oncolytic agents.