1. Antibody toward phenylalanine ammonia-lyase of root tissue of sweet potato injured by cutting was obtained by immunizing of a rabbit with purified enzyme. 2. Phenylalanine ammonia-lyase activity developed markedly in disks of sweet potato roots in response to cut injury, reached a maximum after 12 h, and then decreased thereafter. Phenylalanine ammonia-lyase content, which was determined by the quantitative immunoprecipitation method, changed in parallel with enzyme activity. The rate of incorporation of the label from [14C]leucine into phenylalanine ammonia-lyase initially increased with the time of incubation after slicing, reached a maximum in 6h of incubation, then remained at an almost constant level regardless of the decrease in enzyme activity. The results indicate that the increase in enzyme activity is due to de novo synthesis of phenylalanine ammonia-lyase and the subsequent decrease in activity is not based on decreased synthesis of phenylalanine ammonia-lyase.