A novel quantitative gene-locus mutation assay has been developed using a line of human lymphoblast cells, designated AHH-1, competent in oxidative xenobiotic metabolism. AHH-1 cells are sensitive to the mutagenic action of both chemically reactive mutagens and mutagens which require oxidative metabolism to exert their mutagenicity. These cells are readily mutated by direct exposure to ethyl methanesulfonate, ICR-191, 2-acetoaminofluorene, aflatoxin B1, benzo[a]pyrene (BP), cyclopenta[c, d]pyrene, dimethylnitrosamine, lasiocarpine, and 1-methylphenanthrene.