In this work, we studied the development of tolerance to low-intensity violet (405 nm, 80 mW/cm2 , 72 J/cm2 ) LED radiation in a clinical antibiotic-resistant strain of Staphylococcus aureus 2a. The change in numbers during 20 cycles of irradiation was studied, the reaction of bacterial cells to oxidative stress was studied – sensitivity to the presence of hydrogen peroxide in the environment and catalase activity. It was shown hat from cycles 1 to 5 there was a signifi cantly insignifi cant reduction in survival rate – from 85% to 82%, from cycles 5 to 10 of irradiation the decrease in the number of cells became more pronounced – from 82 to 63%, then, from cycles 10 to 15, recovery was noted, showing numbers to higher values (65–76%), from cycle 15 to 20, the survival value after irradiation remained at the same level (80%). It has been established that, starting from the 15th irradiation cycle, the culture becomes 2 times more resistant to the action of oxidative factors. The results obtained showed that the use of the photodynamic therapy method in practice must be approached with caution, since the formation of tolerance to the eff ects in the tagret microorganism occurs by the 15th irradiation cycle and can signifi cantly worsen the treatment results.