In the absence of a monooxygenase system, mutations induced by benzo (a)pyrene 4,5-oxide are readily quenched by epoxide hydrase, whereas mutations induced by a diol epoxide metabolite of benzo (a)pyrene [(+/-)-7 beta, 8alpha-dihydroxy-9beta, 10beta-epoxy-7,8,9,10-tetrahydrobenzo (a)pyrene] are not.