This study was undertaken to examine the adaptive response in Chinese hamster V79 cells using the cell survival (colony formation assay) and DNA synthesis ([3H]thymidine incorporation) as endpoints. When V79 cells were preirradiated with an adapting dose (0.05 Gy), their survival after irradiation with the challenging dose (4 Gy) was about 120% of the control without such preirradiation. Following irradiation with the challenging dose, the DNA synthesis of the preirradiated cells was less reduced than those without it. The adaptive responses were considered to be associated with the signal transduction via protein kinase C from the results that this response was not observed when the cells were preirradiated with the adapting dose in the presence of protein kinase C inhibitor, H-7.