There will be described in this paper some experiments on the inactivation of bacteria and bacteriophage by x-radiation and incorporated radioactive phosphorus under aerobic and anoxic conditions. The results of this work and the published observations of others will be analyzed and compared, and a theory will be proposed of the mechanism involved in the molecular action of x-radiation which results in cell and virus inactivation. The theory is outlined here along with some of its predictions which can be tested. The presence of oxygen during the exposure of cells to ionizing radiation manifests itself by a considerable increase in the inactivation produced by a given dose. A cell is said to be inactivated if it is not able to multiply indefinitely (form a visible colony) under appropriate conditions. Figure 1 shows some results of a typical oxygen-effect experiment with Escherichia coli strain B. In the case illustrated, the initial slopes of the two inactivation curves differ by a factor of about 2.5, i.e., the anoxic sensitivity is 0.4 (1/2.5) as great as the aerobic. The slope of the inactivation curve is a measure of radiosensitivity. The change in slope of the high dose part of the lower curve is due, presumably, to the presence of radioresistant mutants. Some facts concerning the reproductive cycle of bacteriophage are summarized in Figure 2. In the upper left a so-called free phage is illustrated, which for the T even (T2, T4, T6) phages of E. coli is a tadpole-shaped structure with a “head” portion measuring some 65 by 95 millimicrons. This virus attaches itself to the much larger bacterium, forming the phage-host complex or, simply, complex. After adsorption, the phage injects its desoxy-ribonucleic acid (DNA), which amounts to about 50 per cent of the virus by weight, into the cell. The empty protein coat of the phage remains outside the cell and is no longer needed, as it may be mechanically removed without effect on the production of new virus. Phage T2 follows the lytic pathway, and during the so-called vegetative phase many new particles are synthesized—about 60 in twenty minutes under standard conditions—and are released by the lysis of the cell. Phages which are viable are defined for our purposes as those able to infect and lyse cells—that is to multiply. It has been observed by Hewitt and Read (17) that bacteriophage T2 shows no oxygen effect when irradiated in nutrient broth, a protective medium. The inactivation of phage in broth is usually considered to be solely a direct effect (energy deposited by the radiation within the virus), which specifically means that the constituents of the broth by competitive reaction reduce to nil the effects of the radiation products of water on the outer surface of the phage.