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Background. "Hairy" roots can be used for production of plant-derived secondary metabolites, such as flavonoids, which have antioxidant and reducing properties. It seems promising to use the process of silver nanoparticles formation as a method of determining the level of reducing power. This approach allows simul­taneously to obtain nanoparticles with different biological activity and evaluate the reducing potential of dif­ferent plants. Objective. The aim of the study was to determine the dependence of optical properties (ultraviolet/visible spectroscopy, UV-Vis spectra) of solutions of silver nanoparticles obtained using wormwood "hairy" root extracts on the content of flavonoids as compounds with reducing activity. Methods. Four Artemisia tilesii “hairy” root lines from the collection of the Institute of Cell Biology and Genetic Engineering NAS of Ukraine were grown for five weeks in liquid 1/2 MS medium with 20 g/l sucrose. "Hairy" roots were homogenized in 70% ethanol, the extracts were centrifuged, total flavonoid content was determined, and extracts were used for the silver nanoparticles preparation via reduction of Ag+ to Ag0 from AgNO3. UV-Vis spectra at 300–600 nm range were recorded right after colloid solution preparation, in five, and in nine days. Results. Total flavonoid content varied from 4.01 ± 0.39 to 15.37 ± 1.08 mg RE/g FW. The UV-Vis spectra curves of absorption increased with the course of time, mostly from day 0 to day 5. At this period, absorption at 370–500 nm wavelength increased almost twofold. The peak absorption of all samples was detected at 440 nm, and the maximum values at the wavelength of 440 nm correlated with the content of flavonoids. This correlation did not change over time. Conclusions. The optical properties of silver nanoparticles colloid solutions obtained using A. tilesii "hairy" root extracts correlated with the total flavonoid content of the samples. Even though the absorbance of the colloid solutions increased with time, those increases correlated with flavonoid content as well. The determination of the optical properties of AgNPs colloid solution can be used as a convenient way of quickly comparing the reducing ability of extracts both right after the formation of a colloidal solution and after some time of its storage.