Polyunsaturated fatty acid metabolism is governed primarily by two enzymes, prostaglandin H synthase and lipoxygenase. The crystal structure of the metastable product-oxidized purple form of soybean lipoxygenase-3 was determined at 2.0 Angstrom resolution. The data reveal that the chromophore corresponds to an iron-peroxide complex, a potential intermediate in the catalyzed reaction. A significant alteration of the iron site accompanies the formation of the complex. The structure, the first for a fatty acid-lipoxygenase complex, also reveals an unexpected mode of binding, and identifies amino acid residues that may play significant roles in catalysis, regio- and stereoselectivity.