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Summary
Addition of sulphhydril-compounds, freeze-drying and low temperature reduces inactivation by ionizing radiation of plaque-forming ability in T1-phage by various and nearly additive amounts: if combined these changes of circumambient conditions during irradiation lead to ‘factors of protection’ of p = 16 (as compared to irradiation in highly concentrated broth). The results fit a model of radiation effects as proposed by Alexander, Howard-Flanders et al. after its extension to provide for the effects of low temperature. Existing physico-chemical evidence in support of the model is discussed briefly. Replacing some of the thymine in the DNA of T1 by 5-bromouracil leads to a sensitization of inactivation by ionizing radiation which neither depends on nor changes the efficiency of the ‘protective measures’ mentioned above with one exception: when irradiation is done in sulphhydril-containing broth sensitization by 5-bromouracil vanishes completely.