Previous work from this and other laboratories has shown that many unrelated, nonmutagenic physical, chemical, or nutritional stresses increase the resistance of Escherichia coli AB1157 to short-wavelength UV while reducing their UV-induced mutation frequency. The mechanism appears similar in all these cases, suggesting that induction of the same error-free DNA repair pathway is involved. We now show that a combination of two such stresses is no more effective than one alone, in agreement with this hypothesis.