Abstract—Exposure of the thymine requiring bacterium Escherichia coli strain B3 to ultraviolet light (u.v.) prior to incubation in the absence of thymine shortens the lag period normally observed before the onset of death due to lack of thymine. Culture conditions promoting synthesis of new kinds of enzymes at the time of thymineless challenge after u.v. irradiation enhance this effect. The effect can be reversed either by the addition of thymine or photo‐reactivation. Possible mechanisms for these phenomena are discus