The research was carried out in the fruits of four late varieties of plum stored at the end of August 2023 by three methods. Of them, two varieties (Stanley and President) are of foreign origin and two (Udlinennaia and Superpresident) are of domestic origin. In the fruits of these varieties, indicators were dynamically determined: the total content of phenolic substances and the activity of the polyphenoloxidase enzyme. The change in the total content of phenolic substances as well as the activity of the enzyme polyphenoloxidase in plum fruits during storage depended on the intensification of the metabolic processes that took place during maturation in certain periods of storage, the conditions and duration of storage, the storage methods applied, the biological peculiarities of the varieties researched, as well as the action of SBA Reglalg, microelements and CaCl2 used to treat trees during the growing season. Based on the results obtained from the research of the total content of phenolic substances and the activity of the polyphenoloxidase enzyme, we find that in the fruits preserved by the methods of treatment with the ethylene synthesis inhibitor Fitomag and in the controlled atmosphere, the oxidation-reduction processes at full maturation of the fruits took place more slowly. The fruits were kept for a longer period of time (respectively 9; 16 days more) compared to those in the usual atmosphere, preserving their taste qualities and the other characteristic particularities of the variety as well as their resistance to various diseases. In the usual atmosphere, the metabolic processes took place more intensively, producing fruit maturation in shorter terms.