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 repair-proficient 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 efficiently 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.