The metabolic activation or detoxification of mutagens and carcinogens of several chemical classes was investigated in the presence of various rat liver and lung subcellular fractions and of dicoumarol, a specific inhibitor of DT diaphorase activity. His- Salmonella typhimurium strains were used as targets of mutagenicity. Dicoumarol partially prevented the metabolic activation of some promutagens, such as the heterocyclic amines 2-amino-3,4-dimethylimidazo[4,5-f]quinoline and 3-amino-1-methyl-5H-pyrido[4,3-b]indole, and a cigarette smoke condensate. Moreover, detailed experiments, also using purified enzyme, confirmed the participation of DT diaphorase in the metabolic reduction of 4-nitroquinoline N-oxide 4NQO and of hexavalent chromium [Cr(VI)] compounds. The results obtained provide evidence for broad involvement of DT diaphorase in the metabolism of both organic and inorganic mutagens and carcinogens. Moreover, they suggest a dual role of this enzyme, providing not only a cellular detoxifying system but also, with a few substrates, an activating mechanism.