Human tumor cell strains having different activities of O6-alkylguanine-DNA alkyltransferase (ATR) were transplanted into nude mice and chemotherapeutic responses of tumor xenografts were compared after intraperitoneal injection of the anti-tumor drug 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). The tumor strains used were four Mer+ strains possessing high ATR activity and three Mer- strains lacking this activity. Included in these Mer+ strains was a clone 5'dD which expresses the Escherichia coli ATR in Mer- HeLa cells and thus shows the Mer+ phenotype. All the Mer- tumor xenografts were much more sensitive than tumors of Mer+ strains, including the clone 5'dD; after the highest ACNU dose (three injections of 50 mg/kg), some Mer- tumors disappeared completely and the growth of other tumors was severely retarded, whereas all Mer+ tumors continued to grow. These results demonstrate that ATR activity in tumor cells is a major determinant of tumor response to ACNU, and further suggest that measurement of ATR activity in biopsy specimens may provide a useful guide to predict the response to chemotherapy.