Cultured Chinese hamster cells, line M3-1, were exposed to 2537 Å ultraviolet light (UV). Postirradiation growth studies indicated that nonsurviving cells do not divide and that cells surviving a 95% lethal exposure resume growth at a normal rate after a 30-hour lag. Split-dose experiments, in which survival was evaluated on the basis of colony formation, indicated the surviving cells do not repair sublethal injury promptly as with x-irradiation but acquire their original resistance after about 30 hours (in the case of cells surviving a 95% lethal exposure). Some surviving cells developed into small colonies which, when selected and subcultured, were found to grow more slowly and to possess increased sensitivity to x-irradiation; these defective properties persisted for 10 to 20 weeks.

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