[en] Logarithmic phase E. coli cells are much more resistant to X rays if grown in rich medium, e.g., yeast extract-nutrient broth, than if grown in glucose-salts minimal medium. The authors have called this phenomenon medium-dependent resistance (MDR). Their laboratory has also described three kinds of repair for X-ray-induced DNA single-strand breaks: Type I, II, and III. In this work the authors asked how Type II and Type III repair were affected in cells showing MDR. They found that Type II repair (the polA-dependent, rapid repair) was not affected by preirradiation growth conditions, while Type III repair (the recA-dependent, slow repair) was greatly enhanced in cells showing MDR. The authors also showed that the Type III repair of DNA single-strand breaks is, in fact, the repair of DNA double-strand breaks. By following the time-course of double-strand break repair on alkaline and neutral sucrose gradients, they were able to demonstrate a fast-sedimenting species of DNA (neutral gradient) that appears after the strand breaks have been repaired. This suggests two phases of DNA double-strand break repair: first, the X-ray-induced strand breaks are repaired concomitant with the appearance of fast-sedimenting DNA; second, the DNA regains its normal sedimentation characteristics