Etoposide is commonly used in combination with radiation but the best method of integrating these two treatment modalities is not known. In this study, the interaction of etoposide and radiation was investigated using V79 hamster lung fibroblast cells. It was discovered that etoposide (0.25 μg/ml) was a dramatic radiosensitizer if it was given for a 24 hour exposure after radiation. If the cells were exposed to the same concentration of etoposide for 24 hours prior to radiation, minimal or no radiosensitization occurred. Radiosensitization correlated with a prolongation of the radiation-induced G2 cell cycle arrest. These initial studies indicate that the sequence of radiation and etoposide treatments is important in order to maximize cytotoxicity in V79 cells. Further work using human cell lines will be required to determine the clinical applications of this discovery. © 1993.