Aspergillus flavusproduces an adaptive enzyme (or enzymes) that cleaves the heterocyclic ring of 1 mole of quercetin using 1 mole of oxygen to yield 1 mole of carbon monoxide and 1 mole of the depside, 2-(3′,4′-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid. The enzyme acts on chromones that possess a substituent such as a methyl or a phenyl group on carbon 2, a free hydroxyl on carbon 3, and a double bond between carbons 2 and 3. These requirements must also be present in a substrate if production of the enzyme is to be induced. The enzyme was purified 40-fold by treatment with Sephadex followed by adsorption on and elution from diethylaminoethyl cellulose and calcium phosphate gel, then by heating, and finally by precipitation with ammonium sulphate. The enzyme is stabilized towards denaturation by heat, by its substrate, or by low concentrations of ammonium sulphate. It is stable for long periods when lyophilized or stored in solutions at −20 °C. The optimum pH for activity lies between pH 5.0 and 7.0 and maximum stability occurs at pH 6.0.