Cell lines with constitutive levels of the DNA repair protein O6-methylguanine-DNA methyltransferase (O6MeGMT) (Mer+ phenotype) were less sensitive to the cytotoxic effects of the imidazotetrazinone mitozolomide and the methyl analogue (CCRG 81045) than cells lacking the repair enzyme (Mer-). In contrast neither chlorambucil or the ethylimidazotetrazinone (CCRG 82019) showed differential toxicity between Mer+ and Mer- cell lines. When Mer+ cell lines were incubated with the free base O6-methylguanine (O6MeG) for 16 hr there was a depletion of O6MeGMT, which was dose-related. Such cells showed an increased sensitivity to both mitozolamide and CCRG 81045, but not to CCRG 82019. The only Mer+ cell line not showing increased sensitization with O6MeG pretreatment was Raji, where O6MeGMT was shown to reappear after addition of CCRG 81045. These results suggest that the chloroethyl and methylimidazole-triazinones are similar to the nitrosoureas and triazenes in that cytotoxicity correlates with alkylation of the O6-position of guanine, while the ethyl analogues appear to produce an alternate cytotoxic lesion.