Cultures of Escherichia coli will not grow in media containing very high specific activities of radiophosphorus P 32 , the inhibition of growth being due to the decay of assimilated P 32 atoms. Experiments with a differentially labeled thymineless strain of E. coli show that the P 32 disintegrations which occur in the bacterial deoxyribonucleic acid, i.e . in the nucleus, are mainly responsible for the inactivation of the cell. The kinetics with which radioactive bacterial populations are inactivated indicate that the function of several nuclei per bacterial cell must be eliminated by P 32 decay before the ability to generate a colony is lost. The efficiency with which each P 32 disintegration inactivates the nucleus in which it has occurred is calculated to be 0.02 (at –196°), i.e ., similar in magnitude to the killing efficiency of P 32 decay in bacteriophages. P 32 decay and thymine starvation cooperate in bringing about the death of individuals of the thymineless strain, from which observation it is inferred that "thymineless death" is likewise a nuclear inactivation. The descendants of a non-radioactive bacterial culture grown for several generations in the presence of P 32 and the descendants of a radioactive culture grown in the absence of P 32 are inactivated by P 32 decay in a manner which indicates that the phosphorus atoms of bacterial nuclei are dispersed among the progeny nuclei in their line of descendance.

The inactivation of the phages T1, T2, T3, T5, T7, and λ by decay of incorporated P 32 has been studied. It was found that these phages fall into two classes of sensitivity to P 32 decay: at the same specific activity of P 32 in their deoxyribonucleic acid (DNA), T2 and T5 are inactivated three times as rapidly as T1, T3, T7, and λ. Since the strains of the first class were found to contain about three times as much total phosphorus per phage particle as those of the second) it appears that the fraction of all P 32 disintegrations which are lethal is very nearly the same in all the strains. This fraction α depends on the temperature at which decay is allowed to proceed, being 0.05 at –196°C., 0.1 at +4°C., and 0.3 at 65°C. Decay of P 32 taking place only after the penetration of the DNA of a radioactive phage particle into the interior of the bacterial cell can still prevent the reproduction of the parental phage, albeit inactivation now proceeds at a slightly reduced rate. T2 phages inactivated by decay of P 32 can be cross-reactivated; i.e ., donate some of their genetic characters to the progeny of a mixed infection with a non-radioactive phage. They do not, however, exhibit any multiplicity reactivation or photoreactivation. The fact that at low temperatures less than one-tenth of the P 32 disintegrations are lethal to the phage particle and the dependence of the fraction of lethal disintegrations on temperature can be accounted for by the double stranded structure of the DNA macromolecule.