Under conditions of the induced error-prone SOS response in Escherichia coli, N-methyl-N-nitrosourea (MNU) was found to produce a majority of mutations at A.T base pairs. These mutations included mainly A⋅T → G⋅C transitions, followed by A⋅T→T⋅A transversions and (-1)A⋅T frameshifts. The possibility that 3-methyladenine (3-MeAde) significantly contributed to these mutations was investigated. MNU mutagenesis under SOS conditions was studied in E.coli strains deficient in 3-MeAde-DNA glycosylase I (TagI), which is the major constitutively expressed repair enzyme for 3-MeAde. In SOS uninduced cells, the lack of 3-MeAde repair did not increase mutagenesis, suggesting that 3-MeAde does not contribute to mutagenesis under these conditions. In SOS-induced cells, by contrast, MNU induced a 5-fold higher mutation frequency in the TagI-deficient cell strains, suggesting that 3-MeAde is indeed an SOS-dependent premutagenic lesion. Although 3-MeAde is mutagenic under SOS conditions, it is possible that its fast repair in repairproficient cells might result in the loss of the lesion before its mutagenic properties could be realized. Hence, the contribution of 3-MeAde to SOS-dependent mutagenesis in fully repair-proficient cells was also investigated. 3-MeAde lesions were removed from MNU-treated DNA by the purified TagI protein. This prior removal of 3-MeAde had little effect on mutagenesis in SOS-induced or SOS-uninduced cells. Thus, in repair-proficient cells, 3-MeAde is emciently removed from DNA and does not contribute in a major way to mutagenesis. These results indicate that some other A or T adduct(S) are responsible for the bulk of the mutagenesis observed under SOS-induced conditions.