Horseradish peroxidase (HRP), a highly investigated member of the peroxidase family, has been known − among many other biological activities − to catalyze the oxidation of flavonoids and phenolic substrates overall, quercetin among them. On the other hand, quercetin is very well known for its expressed antioxidant activities, which in the case of UV external radiation can be expressed partly in a preventive manner: quercetin is an excellent UV-absorber. Therefore, the aim of this investigation was to study quercetin oxidation by HRP in phosphate buffer, under conditions of UV-stress, i.e. continuous, prolonged UV-B irradiation. The results show that while UV-B irradiation had definitely affected the activity of HRP, and the overal rate of quercetin oxidation by HRP, it had probably very little effect on it for longer UV-B-irradiation periods (>30 min).