The Chinese hamster cell mutant EM-C11, which is hypersensitive to the cell killing effects of alkylating agents compared to its parental line CHO9, has been used to study the impact of base excision repair on the mutagenic effects of DNA methylation damage. This cell line has a defect in the xrcc1 gene. XRCC1 can interact with DNA polymerase-beta, thereby suppressing strand displacement, and DNA ligase III, both of which have been implicated in base excision repair. XRCC1 may, therefore, allow efficient ligation of single-strand breaks generated during base excision repair. Both EM-C11 and CHO9 cells were treated with methyl methanesulfonate (MMS), a DNA-methylating agent reacting predominantly with nitrogen atoms generating adducts which are substrates for the base excision repair pathway. EM-C11 cells are much more sensitive to the cytotoxic effects of MMS than CHO9: for EM-C11, the dose of MMS inducing 10% survival is 6-fold lower than that for CHO9. In contrast, mutation induction at the hprt locus following MMS is similar in EM-C11 and CHO9. Molecular analysis of hprt gene mutations showed that although the largest class of hprt mutations, both in EM-C11 and CHO9 cells, consisted of GC > AT transitions, most likely caused by O6-methylguanine, the size of this class was smaller in EM-C11. The fraction of deletion mutants in EM-C11, however, was twice as large as that found in CHO9 cells. These results suggest that reduced ligation efficiency of single-strand breaks generated during base excision repair, as result of a defect in XRCC1, may lead to the formation of deletions.