The climate in the Republic of Sakha (Yakutia) is sharply continental, characterized by long winters and short summers. It is known that under extreme climatic conditions in plants, the total content and structural diversity of biologically active substances of regulatory and protective action increases. In the experimental study, the plant Phlojodicarpus sibiricus was used, which has been used in ethnomedicine among the peoples of Siberia and Yakutia since ancient times.The leaf extract of the bloater contains phytosterols, which are considered in the work as the active substance under study, belong to the group of steroid alcohols naturally present in plants. The aim of this study was to evaluate the cytotoxic effect on the proliferative activity of cultured HepG2 cancer cells. The measurement of antitumor activity was based on the MTT-assay method, which is an indicator of mitochondrial function in viable cells, based on tetrazolium reduction.o perform this test, HepG2 cancer cells were cultivated at 37°C with 5% CO2 in DMEM medium with the addition of 10% fetal bovine serum until a monolayer was formed. To obtain a cell suspension, the formed monolayer of cells was trypsinized, followed by inactivation of trypsin with the addition of a nutrient medium. A 96-well plate was used to carry out the test. From 2500 to 2800 cells were added to each well in 200 ?l of culture medium. Next, the plates with cells were incubated for 24 hours in a CO2 incubator for cell adhesion at a temperature of 37°C. The reference substance for comparing the antitumor activity of Ph. Sibiricus was the drug Doxorubicin. As a result of the three stages of the experimental study, the most effective concentration of the antitumor activity of the Ph. Sibiricus lyophilisate suspension against cultured HepG2 cancer cells was established, equal to the range from 0.025 g/ml (1:4) to 0.017 g/ml (1:6). Thus, this plant growing in the cryolithozone can be considered as a potential promising medicinal plant material with antitumor activity, further more in-depth study is necessary.