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Sweetclover (Melilotus alba Desr.) contains a soluble enzyme which oxidizes phenylpyruvate with the addition of either a natural cofactor or Mn2+ and dichlorophenol. The final product of oxidation has not been identified. Phenylpyruvate oxidation is catalyzed also by horseradish peroxidase, and the oxidation is enhanced by the natural cofactor from sweetclover or Mn2+ and dichlorophenol. After fractionation of sweetclover homogenates by DEAE-cellulose chromatography, two peaks of peroxidase activity (pyrogallol as substrate) are apparent; both peaks also oxidize phenylpyruvate. A third peak, specific for phenylpyruvate oxidation, also is apparent. The Cu/cu and B/b allelic pairs, which influence phenolic metabolism in sweetclover have little, if any, influence on peroxidase activity. An autoxidation of phenylpyruvate occurs at high Mn2+ concentrations.