Agaridoxin, a strongly autoxidizable substance, was isolated from the common mushroom Agaricus campestris. Its structure was established, largely by NMR, mass spectroscopy, uv spectroscopy, and polarography, to be 3,4 dihydroxy-(γ-L-glutamyl)anilide (1). This metabolite was synthesized starting with the reaction between 3,4-(isopropylidenedioxy)aniline (5) and N-phthaloylglutamic anhydride (6). The resulting substituted phthalimide (7) was converted into (γ-L-glutamyl)-3,4-(isopropylidenedioxy)anilide (9) by treatment with hydrazine in ethanol. Removal of the isopropylidene protecting group by use of boron trichloride gave agaridoxin in good yield.