When the onion extract that had been treated at a high temperature was added, the banana polyphenol oxidase activity was markedly inhibited. It was found that heat treated onion extract inhibited the banana polyphenol oxidase non-competitively. The MRP synthesized from arginine, cysteine, histidine and lysine significantly inhibited banana polyphenol oxidase. The enzyme activity was inhibited by addition of various anti-browning agents.