The X-ray-induced inactivation of the biological activity of Bacillus subtilis transforming DNA in dilute aqueous solution has been studied over a wide range of O2 concentrations in an attempt to elucidate the mechanisms involved in O2 action. When the DNA is irradiated in the presence of 100 per cent O2 there is a protection of the transforming DNA compared to the sensitivity in N2-saturated or in N2O-saturated solutions. When the equilibrating gas contains intermediate concentrations of O2 (1 per cent--90 per cent) in N2 or N2O, the DNA sensitivity is equivalent to that in pure N2 or N2O respectively. At low O2 concentrations (approximately 0.14 per cent O2 in N2 or in N2O) there is a sensitization of the DNA and this sensitization can be prevented by .OH scavengers. Possible mechanisms for these actions of O2 on the radiation sensitivity of transforming DNA are discussed.