We have already reported that the quinol formation from some para-alkylphenols, which is a novel metabolic pathway catalyzed by cytochrome P-450, occurs in a rat liver microsomal system (). In the present study, we investigated whether estrone and 17beta-estadiol, each of which contains a p-alkylphenol moiety, are also oxidized into the corresponding quinols by cytochrome P-450. Six recombinant human cytochrome P-450 enzymes, CYP1A1, CYP1A2, CYP2B6, CYP2C9, CYP2E1, and CYP3A4, were tested. The results show that estrone and 17beta-estadiol were converted into the corresponding quinols by CYP1A1, CYP2B6, and CYP2E1.