The degradation of DNA in a repair-proficient strain (B/r) and in two repair-defective strains (Bs–1and pol A) ofEscherichia coliwas studied in the presence and absence of three metabolic inhibitors after exposure of the cells to X-radiation. The radiation-induced degradation of DNA was dependent on energy production in freshly harvested log-phase cells and was closely related to the repair capacity of the cells. A different system, probably involving nonspecific deoxyribonucleases acting at radiation-induced strand breaks, appeared to be responsible for the degradative processes in stationary-phase cells or in log-phase cells which had been aged by standing in buffered saline for 2 days before irradiation. The three inhibitors tested (caffeine, rifampin, and carbonyl cyanidem-chlorophenyl hydrazone) were all found to inhibit partially the radiation-induced degradation of DNA inE.colicells. Under other conditions, the same three compounds amplified the degradative process. The data suggested that the amplification was due to a selective inhibition of repair processes. The degradative processes which were stimulated by X-radiation ofE.colicells show many parallels to those which are evoked by phleomycin or colicine.