The consumption of red meat is a risk factor in human colorectal cancer (CRC). One hypothesis is that red meat facilitates the nitrosation of bile acid conjugates and amino acids, which rapidly convert to DNA-damaging carcinogens. Indeed, the toxic and mutagenic DNA adduct O6-carboxymethylguanine (O6-CMG) is frequently present in human DNA, increases in abundance in people with high levels of dietary red meat and may therefore be a causative factor in CRC. Previous reports suggested that O6-CMG is not a substrate for the human version of the DNA damage reversal protein O 6-methylguanine-DNA methyltransferase (MGMT), which protects against the genotoxic effects of other O6-alkylguanine lesions by removing alkyl groups from the O6-position. We now show that synthetic oligodeoxyribonucleotides containing the known MGMT substrate O 6-methylguanine (O6-MeG) or O6-CMG effectively inactivate MGMT in vitro (IC50 0.93 and 1.8 nM, respectively). Inactivation involves the removal of the O6-alkyl group and its transfer to the active-site cysteine residue of MGMT. O6-CMG is therefore an MGMT substrate, and hence MGMT is likely to be a protective factor in CRC under conditions where O6-CMG is a potential causative agent. © The Author(s) 2013.