Methyl excision repair deficient human tumor cells (Mer-) were found to be hypersusceptible to killing by the antimelanoma agent fotemustine (FM) implicating alkylation of O6 guanine as the major contributor to toxicity. Preincubation of the drug in aqueous solution for 5 min resulted in an immediate reduction in cytotoxicity (35-50%), in vitro DNA alkylation (31%), and DNA interstrand cross-linking (40%) followed by a second reaction with considerably slower kinetics. Electrospray ionisation mass spectrometry (ESI-MS) showed that in aqueous solution FM rearranged rapidly to form either a metastable tautomer or decomposed to form a highly reactive diazohydroxide (t1/2 < 2 min). These results suggest the presence of two DNA-reactive species relevant to biological activity. Coincubation of ellagic acid (an inhibitor of O6-guanine alkylation) with FM inhibited in vitro ISC, suggesting that the O6-chloroethyl lesion is the predominant cause of the cross-link. On the basis of these findings, we propose that FM breaks down to form a short-lived intermediate, 2-chloroethyldiazohydroxide, which rapidly generates O6-guanine lesions responsible for the drug's initial activity and a long lived iminol tautomer responsible for the remaining O6 guanine alkylation and cytotoxicity.