Scientific research was carried out in the leaves of four late varieties of plum, two of foreign origin (Stanley and Prezident) and two of local origin (Udlinenaia and Superprezident). In them was determined the activity of the enzymes peroxidase and catalase. The activity values of the investigated enzymes during the vegetation period changed depending on the metabolic processes in the leaves of the trees that took place in certain phenophases, on the environmental conditions, on the amount of oxidative substances that formed in plant cells under the action of high temperatures, the biological peculiarities of the varieties, as well as the substances used to treat plum trees. In the version of the trees treated with SBA Reglalg and microelements B, Zn, Mn, Mo, the activity of these enzymes was higher than in the control version. The mentioned substances positively influenced the activity of these enzymes during the vegetation period. With the modification of the activity of the enzymes peroxidase and catalase in ontogenesis, we mention their participation in the metabolic processes that correlate with the functions of resistance and adaptation of plants to unfavorable environmental conditions and, consequently, the formation of the harvest and the quality of the fruits.