Diamine oxidase (EC 220.127.116.11) was purified from 5-day-old etiolated seedlings ofLathyrus sativus by MnCl2 treatment, (NH4)2SO4 and acetone fractionations, DEAE-Sephadex chromatography followed by gel filtration on Sephadex G-200. A single step purification of the enzyme was achieved by using an immunoaffinity column, wherein rabbit antibodies to the homogeneous diamine oxidase were coupled to CNBr-activated Sepharose. The enzyme thus obtained was homogeneous by electrophoretic, immunological and ultracentrifugal criteria. It had anMr of 148,000 (6.46S) and was a dimer with similar sub-units (Mr 75,000). Amino acid analysis showed the absence of cysteine residues although it contained five disulphide bonds. The enzyme had copper (2.7 g atom/mol enzyme) but was not a glycoprotein. No absorption maximum in the visible region was detectable. Ethylenediamine 1,3-diaminopropane and histamine were potent competitive inhibitors for the substrate putrescine. The addition of monospecific antibodies to the enzyme increased the Km for benzyl amine without any change in the Vmax Diamine oxidase from pea seedling, partially purified, exhibited complete crossreactivity with the antibodies to theL. sativus enzyme.