Enzymes have been used extensively in many industries for the last 20 yrs. The purpose of this study was the isolation, purification, and specification of diamine oxidase (DAO) of pea seedlings. The relationship between enzyme activity and growth conditions has been investigated. DAO that was extracted from pea seedlings was purified by centrifugation, thermal denaturation, fractionation with ammonium sulfate, precipitation of inert components, column electrophoresis, and DEAE-cellulose column chromatography. It was found that the final enzyme preparation is 400-fold purer than the original extract at the end of the purification steps. The molecular weight, isoelectric point, and copper content of the purified enzyme also were determined.