Cultured human kidney T1 cells were allowed to recover between paired doses of X-rays and accelerated heavy ions. Recovery from sublethal doses of X-rays and heavy ions having LET's up to$1650\ {\rm MeV}\text{-}{\rm cm}^{2}/{\rm gm}$ was evident on the basis of colony formation. No recovery could be detected between paired doses of accelerated ions having higher LET's, and very little between paired 150-rad doses of X-rays. Survival curves for colony formation were also determined; recovery occurred whenever survival curves were "sigmoid" and was absent whenever survival curves were exponential. There was no obvious reduction in the extent of recovery at intermediate LET's, although it may have been slightly delayed. The results indicate that exponential survival curves correspond to irreversible inactivation and "sigmoid" survival curves correspond to lethality due to the accumulation of reversible sublethal lesions. The probability of the former appears to increase more rapidly with LET than that of the latter.

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