NADPH-fortified human liver microsomes were examined with regard to ability to detoxicate several chemicals that do not require enzymatic oxidation to elicit a genotoxic response in a Salmonella typhimurium TA1535/pSK1002 system where umu response is used as an indicator of DNA damage. Microsomes did not affect the response seen with daunomycin, mitomycin C, 2,4,7-trinitro-9-fluorene, 1-nitropyrene, doxorubicin, 1-methyl-3-nitro-1-nitrosoguanidine, 2-nitrofluorene, or 1-ethyl-3-nitro-1-nitrosoguanidine (cited in order of decreasing umu response per mol). Human and rat liver microsomes did inactivate 1,3-, 1,6-, and 1,8-dinitropyrene; with human liver microsomes the activity of 1,3-dinitropyrene was most strongly inhibited, while with rat liver microsomes the genotoxicities of all three dinitropyrenes were inhibited to a similar extent. NADPH-cytochrome P-450 reductase was demonstrated to inactivate 1,6- and 1,8-dinitropyrene but not 1,3-dinitropyrene. Both rat cytochrome P-450 beta NF-B (P-450 IA1) and P-450ISF-G (P-450 IA2) inactivated 1,3-dinitropyrene, with the former being more effective. Correlation studies done with liver microsomes prepared from variously treated rats and immunoinhibition studies suggest that cytochrome P-450 beta NF-B and P-450ISF-G are both involved in the detoxication of all three of the dinitropyrenes in rat liver microsomes. In a series of assays done with various human liver microsomal preparations, the inactivation of the three dinitropyrenes was not correlated to each other at all. Correlation analysis and inhibition studies with 7,8-benzoflavone and antibodies indicate that human cytochrome P-450 enzymes in the IA family are most effective in detoxicating this compound; the contribution of cytochrome P-450PA (P-450 IA2, the phenacetin O-deethylase) is deemed more important, but a role for the small amount of cytochrome P1-450 (P-450 IA1) in the liver cannot be ruled out. In contrast to the case of 1,3-dinitropyrene, the inactivation of 1,6-dinitropyrene is well correlated with levels of cytochrome P-450NF (P-450 IIIA4, nifedipine oxidase) and its catalytic activities. The inactivation of 1,8-dinitropyrene was not correlated with any of the above parameters and only correlated with the conversion of benzo(a)pyrene to its 3-hydroxy and 4,5-dihydrodiol products, for which the principal enzymes involved in human liver are unknown. Thus, distinct human cytochrome P-450 enzymes are involved in the detoxication of different dinitropyrene congeners, and the situation appears to contrast with that in rat liver.