Thiotepa (N,N',N'-triethylenethiophosphoramide) is an alkylating agent used in cancer chemotherapy. A reaction pathway by which thiotepa alkylates purified DNA involves hydrolysis to aziridine, which forms N7-aminoethyl guanine and aminoethyl adenine. These lesions are repaired in Escherichia coli by the formamidopyrimidine-DNA glycosylase (Fpg) protein. To determine whether such lesions are formed by thiotepa in mammalian cells, we have overexpressed the E. coli Fpg protein in Chinese hamster ovary cells. The transfected cells were more resistant to the lethal and mutagenic effects of thiotepa than the parental cells. The number of replication-blocking lesions formed by thiotepa, measured by quantitative PCR analysis, was lower in the transfected cells. These results show that expression of the Fpg protein increases the cell resistance to thiotepa and suggest that this compound produces ring-opened guanines, which are involved in its cytotoxic action.