DNA irradiated in cells has previously been showed to be more sensitive in the presence of oxygen. It is shown that enzymes in cells are similarly 3 times as radiosensitive under oxygenated conditions. A paradox then arises, since dilute solutions of enzymes and of DNA have been repeatedly shown to be without an oxygen effect, yet much of the radiation inactivation in a cell is caused by radical attack. It is shown that the radiosensitivity of DNA in preparations of lysed cells is oxygen-dependent. Furthermore, dilute solutions of DNA and of trypsin show an oxygen effect when SH-containing compounds are added. Possible mechanisms for the oxygen effect are discussed in light of these results.