A novel enzyme responsible for the oxidation of spermidine and spermine has been found in rat liver. Spermidine is shown to be degraded to putrescine and 3-aminopropionaldehyde, and spermine to be cleaved to spermidine and 3-aminopropionaldehyde. A single enzyme catalyzing both reactions and designated as polyamine oxidase has been purified 4000-fold to electrophoretic homogeneity. Polyamine oxidase appears to be a flavoprotein, containing flavin adenine dinucleotide (FAD) as a prosthetic group. Hydrogen peroxide is evolved in the reaction and no other electron acceptors except molecular oxygen have been found. The molecular weight of the enzyme was approximately 60 000 and the sedimentation coefficient 4.5 S. The enzyme appears to be a single polypeptide chain since no evidence for structural subunits was obtained. Polyamine oxidase was sensitive to sulfhydryl and carbonyl group reagents. The optimum pH value for the oxidation of polyamines was close to 10. The reaction velocities were enhanced by various aldehydes, especially certain aromatic aldehydes. Polyamine oxidase appears to be localized in peroxisomes of liver cells, although the existence of an isoenzyme in the cytosolic fraction was not definitively ruled out. No marked changes were observed in the activity of polyamine oxidase in rat liver after partial hepatectomy, carbon tetrachloride poisoning, and after treatment with growth hormone or thioacetamide, conditions which are known to alter profoundly the metabolism and accumulation of polyamines.