A procedure for obtaining crystalline preparations of the amine oxidase of Aspergillus niger has been developed. The method involved fractionations with ammonium sulfate and separation on successive columns of DEAE-cellulose, DEAE-sephadex and hydroxyl-apatite. Crystals were formed when solid ammonium sulfate was added to solutions of the purified enzyme (of about 300- to 350-times the specific activity of the crude mycelium extract). The crystals appeared as fine rods, with a faint pink color. The crystals had a specific activity around 10, 000 which was about 20 times as active as that of crystalline preparations of the animal monoamine oxidase.

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