Diamine oxidase (DAO) is a member of the class of copper-containing amine oxidases and catalyzes the oxidative deamination of histamine and other biogenic amines. The enzyme from porcine kidney was purified by consecutive chromatography on concanavalin A Sepharose, heparin Sepharose and Mono Q. Besides being simpler and faster than previous methods, this new purification scheme results in a homogenous product with a considerably higher yield and allows the rapid purification of large amounts of DAO from mammalian tissues. The availability of sufficient pure protein will greatly facilitate future studies of the structure and function of the enzyme.