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Summary
A radiosensitive strain of the mouse lymphoma L 5178Y grown in vitro was exposed to continuous gamma-radiation at dose-rates of 3·0 and 6·3 r/hr. At 6·3 r/hr the number of living cells decreased, but at 3·0 r/hr the culture continued to grow exponentially, although more slowly than normal. Irradiation was continued for a period of over 10 months, during which time the culture showed no signs of deterioration in so far as ability to proliferate was concerned. It was concluded that, for this rapidly-dividing culture, there was no accumulation of radiation damage.
The radiosensitivity of the culture was tested at 1½ and 3 months from the start of irradiation and was found to be unchanged, but at 6 months the culture had become resistant. Growth-rate measurements on the resistant culture suggested that under conditions of continuous irradiation the cell-cycle time was prolonged.