AbstractFive isoflavones, biochanin A, genistein, formononetin, daidzein and 5–7‐dihydroxyisoflavone have been synthesized. Their estrogenic activity was assayed by the mouse uterine weight method under different administration routes. Daidzein, in contrast to the four other isoflavones, was not absorbed, when injected subcutaneously and its estrogenic activity was in consequence of this negligible by subcutaneous injection. By oral or intraperitoneal administration the estrogenic activity of daidzein and genistein was approximately 10‐5times that of diethylstilbestrol. Biochanin A and 5–7‐dihydroxy isoflavone were about half as active estrogens as daidzein and genistein. Formononetin was inactive. A free para‐hydroxy group in the phenylnucleus of the molecule is present in daidzein and genistein but is lacking in biochanin A, formononetin and 5–7‐di‐hydroxy isoflavone. The presence of a free para‐hydroxy groups therefore seems to be of importance for the