Recovery from and fixation of potentially lethal damage after exposure of Chinese hamster cells to uv and to X-irradiation were investigated, as was recovery after exposure to chemotherapeutic agents. Recovery after uv radiation has a <tex-math>$T_{{\textstyle\frac{1}{2}}}$</tex-math> of about 20 hr; the fraction of cells able to undergo recovery depends upon nutritional factors both before and after exposure. After X-irradiation, recovery proceeds with a <tex-math>$T_{{\textstyle\frac{1}{2}}}$</tex-math> of ∼2 hr and is much less influenced by nutritional factors. Fixation after serum stimulation has a <tex-math>$T_{{\textstyle\frac{1}{2}}}$</tex-math> of 3-4 hr in uv-irradiated cells, a <tex-math>$T_{{\textstyle\frac{1}{2}}}$</tex-math> of 30 min in X-irradiated cells. Recovery kinetics after nitrogen mustard and bleomycin exposures mimic those for X-ray exposure; after methyl methane sulfonate the kinetics are mainly uv-like, though with an X-ray-like component. Recovery by cells with BUdR-substituted DNA and irradiated with visible light is primarily X-ray-like, though with a uv-like component. There is no recovery by cells exposed to adriamycin or to 1,3-bis(2-chloroethyl)-1-nitrosourea.

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