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The capacity of oxidation of benzo(a)pyrene (BP) and its analog to be oxidized by peroxidases in several tissues has been studied. The kinetics of the horseradish peroxidase (HRP) oxidation of BP and 7,8-dihydro-7,8-dihydroxy benzo(a)pyrene (BP-7,8-diol) were examined. Effective ratios of H2O2 and HRP for catalytic oxidation were 13.74 for BP and 4.58 for BP-7,8-diol. The maximum ratio was approximately 90 for both hydrogen donors (BP and BP-7,8-diol) to the ES complex. The maximum ratio of oxidized BP and BP-7,8-diol to HRP was 5.7. Ks values for H2O2 were 1.68 and 6.35 microM for BP and BP-7,8-diol, respectively. The mean values of the rate constants, k5, for the oxidation of BP and BP-7,8-diol were 0.56 X 10(5) M-1 sec-1 and 4.1 X 10(5) M-1 sec-1, respectively, at low concentrations. At low concentrations a Hill plot of the oxidation of BP showed a negative value (nH = 0.5) and at high concentrations nH = 1.0. On the other hand, that of BP-7,8-diol showed positive cooperativeness (nH = 1.8). These oxidation reactions caused substrate (donor) inhibition at high concentrations. The inhibition constants, KA', were 9.8 and 5.65 microM for BP and BP-7,8-diol, respectively. The reactivity of the oxidation of BP-7,8-diol was five to six times larger than that of BP.