Isoflavonoid phytoestrogens and lignans in plants are known to be constituents of animal and human food and recently they have been found in human urine and other biological materials. These compounds have received increasing attention because of their interesting biological properties and possible role in human cancer and other diseases. The present study demonstrates that the main mammalian lignan enterolactone (trans-2,3-bis[(3-hydroxyphenyl)methyl]-butyrolactone) and some other diphenols are moderate or weak inhibitors of human estrogen synthetase (aromatase) and that this lignan binds to or near the substrate region of the active site of the P-450 enzyme. The inhibition is competitive with respect to testosterone and androstenedione, and the lignan affinity is 1/75-1/300 that of these natural substrates. It is suggested that the high concentration of lignans in vegetarians, by inhibiting aromatase in peripheral and/or cancer cells and lowering estrogen levels, may play a protective role as antipromotional compounds during growth of estrogen-dependent cancers.