O6-Alkylguanine-DNA alkyltransferase (MGMT) confers resistance to many of the mutagenic and toxic effects of certain classes of alkylating agents by repairing the DNA lesions responsible. The levels of expression of this protein are of interest in relation to the prevention and treatment of cancer in man. They vary widely between individuals, and the basis of this variation is not understood. RT-PCR-RFLP analysis of mRNA from normal human lung tissue reveals that the two MGMT alleles are frequently expressed at different levels, indicating that there is a genetic component to inter-individual variation of MGMT levels and that at least some of this variation maps close to or within the MGMT locus.