A study was made of the regularities of structural chromosome damages in human peripheral blood lymphocytes during irradiation at the G1 stage with 252Cf and 60Co sources in dose rates of 0.035 and 0.35 Gy/min at doses of 0.27-3.1 Gy. In 252Cf radiation, the frequency of chromosome aberrations at the G1 stage was described by linear dose-effect ratio in both dose rates and did not depend on a dose rate in the studied dose range. The RBE of 252Cf radiation at the G1 stage grew with a decrease in a dose and dose rate. Postradiation hyperthermia caused less noticeable enhancement of the effect of 252Cf radiation (approximately 10%) as compared to that of 60Co radiation (approximately 30%).