The use of the spin traps nitrosobenzene and 2-methyl-2-nitrosopropane has established that metmyoglobin and liver microsomal cytochrome P-450 initiate a radical decomposition of cumene hydroperoxide. With metmyoglobin and the alkyl nitroso compound, the only radical product of cumene hydroperoxide trapped was the methyl radical formed by β scission of the cumyloxy radical. With both hemeprotein initiators, nitrosobenzene trapped only the cumyl radical, considered to be a decomposition product of the unstable spin adduct phenylcumyloxynitroxide. Support for this proposal includes: (1) previous spin trapping studies of the chemical decomposition of cumene hydroperoxide; and (2) significant inhibition by nitrosobenzene of the one-electron oxidation of aminopyrine and the autoxidation of unsaturated membrane lipids resulting from addition of the hydroperoxide to liver microsomes. Aminopyrine altered the epr signal amplitudes of the spin adducts produced with both nitroso compounds, indicative of oxidation of aminopyrine by the methyl radical and reduction of cumene hydroperoxide by the aminopyrine radical. The participation of hydroperoxide-derived radicals in the low peroxidatic activities of certain hemeproteins is quite distinct from the catalytic function of the true hemeprotein peroxidases, which bring about an efficient two-electron reduction of specific hydroperoxides.