A new hypocholesterolemic substance, eritadenine, was isolated from an edible mushroom 'Shiitake' (Lentinus edodes), and its chemical structure was confirmed in our laboratory to be 4-(6-amino-9H-purine-9-yl)-2,3-dihydroxybutyric acid (II). Eritadenine is the first example of adenine 9-substituted with a hydroxy acid in natural products, and its biosynthesis is of special interest in our investigation. In view of the structural relationship of eritadenine to adenine, adenine is presumed to be a possible precursor of eritadenine. To prove this hypothesis, adenine-8-C14 was infused into a Shiitake. From the distribution of C14, it was suggested that eritadenine was synthesized from adenine via the intermediate, substance A. This intermediate was obtained as a crystalline powder and identified as 4-(6-amino-9H-purine-9-yl)-3-hydroxybutyric acid (I). SAITO et al. and TOKITA et al. isolated the same substance, but its biological role in Shiitake has not been discussed.