Paranitroacetophenone (PNAP) and triacetoneamine N oxyl (TAN), which sensitize anoxic cells to ionizing radiation, do not increase the radiosensitivity of bacteriophage DNA irradiated in the dry state, showing that the 'direct action model' of sensitization does not hold in this experimental set up. PNAP and TAN added within 1.5 msec after irradiation of the DNA in buffer, do not decrease the survival of biological activity, showing that any reaction between these compounds and 'long lived' DNA radicals is not lethal. When DNA is irradiated in solution in the presence of TAN part of the damage in DNA is reparable by the excision repair system of Escherichia coli. Such an effect is not observed with PNAP. In protective media PNAP and TAN also behave differently with regard to the inactivation of phage T1. Like oxygen PNAP protects the phage a bit and decreases the difference in survival on a Uvr+ and a Uvr- strain of E. coli. TAN sensitizes T1 and increases the difference on the 2 strains. Induction of mutations in phage by γ radiation which is often enhanced by O2, is not significantly affected by the presence of the sensitizers.