We showed that injection of IL-1β (Betaleukin) in a dose of 3 μg/kg 22 h before prolonged (21 h) exposure to low-intensity (10 mGy/min) γ-radiation in a dose of 12.6 Gy reduced the number of double-strand DNA breaks in murine spleen cells to the control level in 4 months after exposure and the number of double-strand DNA breaks induced by additional acute irradiation in a dose of 6 Gy. The results suggest that IL-1β can improve the efficiency of systems reducing the number of double-strand DNA breaks in murine spleen cells at delayed terms after exposure to prolonged low-intensity radiation.