Glutathione S-transferases (EC 2.5.1.18) in mammalian cells catalyze the conjugation, and thus, the detoxication of a structurally diverse group of electrophilic environmental carcinogens and alkylating drugs, including the antineoplastic nitrogen mustards. We proposed that structural alteration of the nonspecific electrophile-binding site would produce mutant enzymes with increased efficiency for detoxication of a single drug and that these mutants could serve as useful somatic transgenes to protect healthy human cells against single alkylating agents used in cancer chemotherapy protocols. Random mutagenesis of three regions (residues 9-14, 102-112, and 210-220), which together compose the glutathione S-transferase electrophile-binding site, followed by selection of Escherichia coli expressing the enzyme library with the nitrogen mustard mechlorethamine (20-500 microM), yielded mutant enzymes that showed significant improvement in catalytic efficiency for mechlorethamine conjugation (up to 15-fold increase in kcat and up to 6-fold increase in kcat/Km) and that confer up to 31-fold resistance, which is 9-fold greater drug resistance than that conferred by the wild-type enzyme. The results suggest a general strategy for modification of drug- and carcinogen-metabolizing enzymes to achieve desired resistance in both prokaryotic and eukaryotic plant and animal cells.