AbstractOestrogen‐dependent breast cancers are considered to produce oestradiol in the breast through the initial action of a sulphatase on the oestrone sulphate reservoir, followed by reduction of the oestrone formed by a 17β‐hydroxysteroid dehydrogenase. The low rate of incidence of the disease in the Far East has been related to a high dietary intake of soya products containing flavones and isoflavones.We have found that the flavones chrysin (IC50 60‐5 μM), apigenin (18‐3 μM) and naringenin (19‐9 μM), and the isoflavones genistein (195 μM), biochanin A (47‐6 μM) and daidzein are potent inhibitors of human placental 17 β‐hydroxy s