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Aim. Research into the change in enzymatic indicators of cell activity during the aging of human dermal fibroblasts in culture from 3 to 15 passages to determine the most optimal terms for cell transplantation to patients for further cell therapy. Methods. Fibroblasts were obtained from the dermis of donors A, B, and C aged 40 to 60 years and cultured in MEM Alpha сulture medium containing 5% FBS, 1% antibiotic/antimycotic, 5ng bFGF until they reached 3, 6, 9, 12, and 15 passages (s1p3-s1p15). When fibroblasts that had reached the required passage were removed, the cells were washed with saline solution and precipitated on a centrifuge. Fixation was carried out with a 0.1% solution of formalin. At the end of the fixation time, the cells were centrifuged and left in 70% ethanol at –20 °C. At least after 3 hours, the cells were washed twice with saline and centrifuged. To determine number of cells at stages of the cell cycle by the quantitative content of DNA, fibroblasts were stained with propidium iodide/RNase buffer. At the end of the sample preparation, the sample was analyzed on a flow cytometer (Beckman Coulter, China). To study the enzymatic activity of mitochondria, fibroblasts were seeded in a sterile 96-well microplate and cultured for 72 hours. As a control, wells without fibroblasts were subjected to the same manipulations as the experimental wells. After that 0.3 mg/ml MTT was added to the wells. In four hours’ time DMSO and 0.015 mg/ml glycine were added to all wells. The contents of the wells were homogenized and measured with a photometer (BioSan, Latvia) at 490 nm. To determine the activity of lysosomal enzymes of fibroblasts cultured in Petri dishes for 72 hours, cells were washed with saline and fixed with formalin. After the end of fixation, fibroblasts were again washed several times with saline solution and stained with the addition of synthetic dye (1.25% reagent C, 1.25% reagent B, 2.5% X-Gal and 95% X-Gal buffer) for at least 2 hours at 37 °C. Cells were washed with saline from dye residues and photographed the results. The results of the groups were compared using the ANOVA test. Differences were observed at a significance level of P < 0.05. Results and Discussion. It is well known that aging cells can be characterized by cell cycle arrest, affecting the phenomenon of cell proliferation (3). Dermal fibroblasts maintain their mitotic activity even up to the 15th passage (Fig. 1). In addition, the average percentage of fibroblasts that were at the stage of mitosis 58.88±4.49% was higher than the average value of indicators at the stages of interphase (41.17±2.72%). Conclusions. Thus, using various cytochemical methods, it has been proven that the culture of human dermal fibroblasts from donors of the age group from 40 to 60 years maintains stability during their cultivation from 3 to 15 passages. The cells of the culture do not show signs of aging and do not show significant changes in indicators during passaging: a consistently high percentage of mitotically active cells and the absence of a significant increase in the indicators of enzymatic activity of mitochondria and lysosomes. [ABSTRACT FROM AUTHOR]