Summary
Rapid split dose and single dose recovery were examined in exponentially growing and in density inhibited, stationary phase cultures of human diploid fibroblasts. The cultures were exposed to cobalt-60 gamma rays at a dose-rate of 50 rad/s: 0–90 min were allowed between exposures or following a single exposure prior to subculture at low density. A rapid phase of recovery occurred which was of greater magnitude in the plateau phase cultures. Dose-dependent rapid recovery occurred within 2–10 min of the initial exposure, and was greater following single dose versus split dose irradiations. When the post-irradiation temperature was maintained at 37°C, two phases of recovery were observed, one at early times (2–10 min) and one later (30–90 min). No recovery was observed with incubation at 4°C, whereas incubation at room temperature (25°C) allowed rapid recovery (2–10 min) to take place, but not the slower process (30–90 min). The hypothesis that this rapid recovery phenomenon is related to cell-to-cell contact and to repair of potentially lethal damage is discussed.