4.2′.4′-trihydroxy-chalcone-4′-glucoside-[β-14C] (II) is incorporated into the coumarano-coumarine coumestrol (I) in lucerne (Medicago sativa L.) in the same manner as it is into isoflavones. These results lead to the conclusion that the chalcone is a common intermediate in the biosynthesis of both coumestrol and the isoflavones. The dilution of the specific activity of the chalcone upon incorporation into I is somewhat greater than upon incorporation into the isoflavones formononetin and daidzein, which also occur in lucerne.