Diamine oxidase (EC 1.4.3.6) present in extracts of axes of soybean (Glycine max) seedling has been purified to homogeneity using series of ammonium sulfate fractionation, DEAE-cellulose, phosphocellulose, hydroxyapatite and 1,12-diaminododecane agarose column chromatography. The molecular weight estimated by a Sephadex G-200 gel filtration was about 145,000. Sodium dodecyl sulfate polyacrylamide gel electrophoresis gave a single band at the molecular weight of 70,000. The optimum pH for activity was 7.5. Putrescine was the best substrate, and spermidine and spermine also served as substrates in decreasing order. The K_m values for putrescine, spermidine and spermine were 5.3×10^(-4)M, 2.8×10^(-5)M and 6.5×10^(-5)M, respectively.