Induction of forward mutations leading to 8-azaguanine resistance was studied in plateau phase Chinese hamster ovary cells exposed to gamma-radiation. Survival and mutation frequency were assessed immediately after irradiation or after 6 hours of post-irradiation recovery at 37 degrees C. Cells exposed to doses above 4Gy showed appreciable increase in the survival level as a result of recovery from potentially lethal damage. The mutation frequency response curve for cells plated immediately after irradiation displayed an average induction rate of 5 X 4 X 10(-5) per viable cell per Gy in the dose range 4 to 10 Gy. Cells subjected to post-irradiation recovery showed a mutation frequency decrease of 30 to 50 per cent. The kinetics of repair of mutational lesions following exposure to 10 Gy gamma-radiation indicated a sharp decline in mutation frequency during the first hour of post-irradiation recovery and a slow decline thereafter. Analysis of these data suggests that the mutation frequency is significantly lower among the fraction of cells which survive by recovering from potentially lethal damage. The involvement of an error-free pathway that contributes to the repair of an appreciable part of the damage during the post-irradiation recovery period is discussed.