Populations of Chinese hamster cells (line V79-379-A) were harvested from monolayer cultures in which net cell multiplication had ceased. Survival based on colony formation following exposure to 250 kV x-rays was studied in cells harvested from 90- and 135-hour-old monolayers. In populations of cells resuming growth after harvesting from 90-hour monolayers, a peak of radiation resistance was observed during growth-resumption G 1 phase and again during late S phase. Cells harvested from 135-hour monolayers had a longer cell cycle but no wave of radiation resistance during the greatly prolonged growth-resumption G 1 phase. The increased resistance during growth-resumption S phase was found to be related to increased extrapolation numbers of the survival curves, and recovery from sublethal injury occurred in growth-resumption S cells. Cell populations harvested from 135-hour-old monolayers regained their ability to recover from sublethal injury within 2 hours.