Abstract <p>In modern conditions, open and self-organizing biological systems are exposed to a complex spectrum of multi-frequency and modulated electromagnetic radiation of various frequency ranges. This work is devoted to the study of the effect of X-ray irradiation at a dose of 4 Gy on the level of DNA damage in peripheral blood leukocytes of mice pre-exposed to extremely high frequency electromagnetic radiation (EHF EMR) in vitro depending on the carrier and modulating frequencies, exposure duration, intensity and duty cycle of the modulating signal. Using the DNA comet method, it was found that pre-irradiation of cells with electromagnetic radiation of extremely high frequencies leads to a statistically significant decrease in radiation damage to DNA during pulse modulation of radiation with frequencies of 0.1, 1, 16, 32, and 50 Hz. At the same time, irradiation both in the continuous generation mode and in the case of amplitude modulation by a harmonic signal was ineffective. The greatest radioprotective effect was observed at carrier frequencies of 42.2, 51.8, and 61.22 GHz, an exposure duration of 20–30 min, an incident power density of 100–250&#xa0;μW/cm², and a duty cycle of the modulation pulse signal of 2–4. The mechanisms of implementation of the obtained biological effects of extremely high frequency electromagnetic radiation associated with the induction of free radicals are discussed.</p>