Irs1 cells are mutants of the Chinese hamster cell line V79-4, and exhibit cross-sensitivity to various DNA-damaging agents, especially to the alkylating compounds methyl methanesulfonate and ethyl methanesulfonate. To test whether this sensitivity was due to the persistence of alkylated residues in DNA, we have transfected irs1 cells with the pMSG expression vector containing two coding sequences for enzymes of different origin, either the E. coli AlkA gene, coding for 3-methyladenine-DNA-glycosylase II, or rat APDG cDNA, encoding alkylpurine-DNA-glycosylase. The two coding sequences for the repair enzymes were ligated in the pMSG vector, under the control of the MMTV-LTR promoter, which is responsive to glucocorticoid regulation. The presence of the AlkA gene or of the APDG cDNA in the transfected cells was detected by Southern blot analysis and the transcription of these foreign sequences was checked by Northern hybridization of the cellular RNA. The transfected irs1 cells treated with [3H]dimethylsulfate removed the 3-methyladenine residues more efficiently from their DNA than the control cells. Irs1 cells harboring the AlkA or the APDG gene become about 2- and 3-fold more resistant to the toxic effect of methyl methanesulfonate, respectively. However, a 3-fold resistance to ethyl methanesulfonate was only observed in irs1 cells harboring the mammalian APDG cDNA.