We have shown previously that certain alkylation products, or alkylation derived lesions, which induce chromosome aberrations (abs) persist for at least two cell cycles in Chinese hamster ovary cells. The increase in abs in the second cycle after treatment contrasts with the classical observation of reduction in ab yield with successive mitoses following ionizing radiation. Here we present evidence that processing of lesions by mismatch repair is a mechanism for ab induction by methylating agents. Our previous studies implicated O6-methylguanine (O6MeG) as an important lesion in induction of abs, particularly in the second cell cycle after treatment. In the absence of repair of O6MeG by alkylguanine DNA alkyltransferase (AGT), new abs were induced in the second cycle after treatment with e.g. methylnitronitrosoguanidine (MNNG) and methylnitrosourea (MNU). Thus, we hypothesized that abs were produced not by O6MeG or its repair in the first S phase, but by subsequent processing of the lesions. We suggested that after replication proceeded past the O6MeG lesion in the first S phase, inserting an incorrect base on the newly synthesized strand, recognition and repair by mismatch repair in the second S phase led to a chromosome ab. Here we used MT1 cells, a human lymphoblastoid cell line that has a defect in strand-specific mismatch repair. MT1 cells are alkylation tolerant and have a mutator phenotype, compared with their parent line, TK6; both MT1 and TK6 cells lack AGT so do not remove the methyl group from O6MeG. While the initial levels of abs at the first metaphase were similar in MT1 and TK6 cells, ab levels in MT1 cells were greatly reduced in the second and third cell cycles following treatment with MNNG, dimethylnitrosamine and MNU, in contrast with the parent TK6 cells, which had more abs in the second cell cycle than in the first. This supports the hypothesis that repair of mismatched base pairs involving O6MeG is one mechanism for induction of chromosome abs. In contrast to the difference in response to methylating agents between TK6 cells and mismatch repair-deficient MT1 cells, the profile of ab induction by an ethylating agent, ethylnitronitrosourea, was similar in MT1 cells to those for TK6 cells and CHO cells.