A mutant of Escherichia coli B/r designated mfd has drastically reduced ability to exhibit "mutation frequency decline" (MFD) the irreversible loss of potential suppressor mutations which occurs when protein synthesis is briefly inhibited after irradiation with UV. We have found that the initial rate of thymine dimer excision in the mfd mutant is only about one-third that of its mfd+ parent strain after a UV dose of 400 erg/mm-2. The yield of UV-induced Tyr+ revertants is 4-10 times higher in the mfd strain than in the mfd+ strain. This is comparable to the level of UV-mutability in the mfd+ strain in the presence of caffeine, an inhibitor of dimer excision. UV-mutability, prophage induction and Weigle reactivation of irradiated gamma phage occur to a greater extent at low UV doses (10-50 erg/mm-2) in the mfd strain compared to the mfd+ strain. We propose that the slow excision repair in the mfd mutant results in a shift in the induction threshold for these UV-inducible functions toward lower UV doses.