Fully hydrated spores of the highly radioresistant organism, Clostridium botulinum 33A exposed to 0.8 Mrad60 Co showed a characteristic pattern of survival associated with thermorestoration. When temperature during radiation was controlled at 0-100°C, the lowest viable cell count was at 0°C. Above 0°C radiosurvival gradually increased by 1 to 1 1/2 log cycles, reaching a peak at 75-80°C. Above 80°C, a rapid drop in survival occurred presumably caused by heat itself. The general features of the above survival pattern were observed in water saturated with N2 as well as in water saturated with O2. Radio-survival of spores in oxic water was at all temperatures approximately 1 log cycle lower than that in <tex-math>${\rm N}_{2}\text{-saturated}$</tex-math> water although the general features of the survival curves were approximately the same. An analysis of the shapes of survival curves in N2 and O2 suggested that the gradual radioprotection in the temperature range between 0°C and 80°C was due to annealment of lethal radicals in anoxic water, i.e. probably ·OH. The lethal radicals in oxic water (probably <tex-math>$\cdot {\rm HO}_{2}$</tex-math>) appeared not to be annealed appreciably.

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