Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. The enzyme O6- methylguanine-DNA methyltransferase repairs O6-methylguanine as well as a minor methylated base, O4-methylthymine, in DNA. Mouse lines deficient in the methyltransferase (MGMT) gene are hypersensitive to both the killing and to the tumorigenic effects of alkylating agents. We now show that these dual effects of an alkylating agent can be dissociated by introduction of an additional defect in mismatch repair. Mice with mutations in both alleles of the MGMT gene and one of the mismatch repair genes, MLH1, are as resistant to methylnitrosourea (MNU) as are wild-type mice, in terms of survival, but do have numerous tumors after receiving MNU. In contrast to MGMT(-/-)MLH1(+/+) mice with decrease in size of the thymus and hypocellular bone marrow after MNU administration, no conspicuous change was found in MGMT(-/-) MLH1(-/-) mice treated in the same manner. Thus, killing and tumorigenic effects of an alkylating agent can be dissociated by preventing mismatch repair pathways.