Abstract: The technology of radiosoitope indicated that the hydrolysate of Polyphenoloxidase (PPO) from leaves of populus deltoides Ⅰ-69 contained leucine. 3H-leucine could be directly incorporated into the synthesis of PPO from old and young leaves of populus deltoides Ⅰ-69. The DPM per mg protein of purification of labelled PPO from young leaf was enhanced 35 Percent as compared with the control. The DPM per mg protein of purification of labelled PPO from old leaf was enhanced 11 percent as compared with the control. The DPM per unit of young leaf PPO was enhanced 71 percent as compared with that of old leaf. These indicated that young leaves could directly use leucine to synthesize PPO more than old leaves could.