Abstract The properties of a mutant of E. coli K12, URT-43, were investigated. Genetic analysis was also carried out. Cross-sensitivity was checked for 4-nitroquinoline-1-oxide and mitomycin C. In parallel with ultraviolet irradiation, URT-43 was more sensitive to both agents than its wild-type strain. However there was an apparent large temperature effect of the action of 4-nitroquinoline-1-oxide, whereas with mitomycin C the effect was only very small. Reflecting the temperature-dependent ultraviolet sensitivity of URT-43 on aga plates, the irradiated URT-43 recovered well in a liquid medium at 30° but not at 42°. The recovery was shown to require all the substances essential for its normal growth, suggesting that the recovery is accompanied by growth. Irradiated URT-43 was still restorable after photoreactivation to the maximal extent. This was true for the bacteria irradiated at dry-ice temperature in which less photoreactivable injuries would be produced than in the bacteria irradiated at romm temperature. These results, as well as the observation that the extent of recovery in a liquid medium of the bacteria irradiated at low tempwerature was much larger than that of the same bacteria after photoillumination, led the authors to assume that the recovery is due to the repair or to a certain tolerance mechanism for injuries which are not limited to the cyclobutane pyrimidine dimers. the mechanism of this recovery has not yet been elucidated. The mutation of URT-43 was mapped by mating and transduction experiments. The efficiency of mating with an Hfr strain was not greatly affected by low and high temperatures, suggesting that the whole mechanism of recombination is not involved in the temperature-dependent recovery in question. The mutated locus was shown to be closely linked to the malB gene by transduction experiments with P1 phage. The linkage to metA was much weaker, suggesting that the mutation is located in the uvrA gene. Two typical phenotypes of URT-43, namely those with negative host-cell reactivation and temperature-dependent ultraviolet sensitivity, could not be separated in these experiments.