Postirradiation deoxyribo-nucleic acid (DNA) degradation was measured in Escherichia coli 15T-(555-7) (requires thymine, methionine, arginine, and tryptophan) as the loss of radioactivity from cells previously grown in media containing thymine-2-C14. Omission of the required amino acids from the post-irradiation growth medium amplified the loss of DNA and allowed the detection of DNA degradation after exposure of the bacteria to doses as low as 2,000 roentgens. Completion of chromosome synthesis by incubation in the absence of amino acids for 90 minutes before exposure to radiation resulted in the loss of less DNA. Restoration of DNA synthetic capacity in these cultures before irradiation by 45 minutes incubation in the presence of amino acids and the absence of thymine, did not restore the rate and extent of DNA degradation to the type observed in cells removed directly from cultures in logarithmic growth. Bacteria having one strand of DNA substituted with tritiated 5-bromouracil and the other with thymine-2-C14 showed an equal rate of strand degradation after irradiation. A cesium chloride density gradient analysis of the DNAre-maining in irradiated cells after postirradiation incubation did not reveal the presence of any denatured DNA.