Most studies of radioadaptive response (RAR) in vivo have revealed its effective induction both by
chronic and single irradiation of a human organism with low doses. The classical way of the formation of RAR on the human peripheral blood lymphocytes (PBL) was noted as in occupationally irradiated personnel as in residents of areas with increased natural or man-made accidental high background radiation. In some studies, the liquidators of the Chernobyl accident and children living in the Chernobyl pollution zone showed the phenomenon of increasing the radiosensitivity of PBL. A comparative analysis of the influence of two types of radiation (γ and β) with close RBE showed comparable effectiveness to induce an adaptive response in human PBL in the mode of chronic exposure in the long term. Evidence of RAR in vivo was also obtained in the study of DNA end-points and chromosomal damage. In an experiment with the effect on neutrophils of peripheral blood of the elderly people (age 63-84 years) induced by acute -rays in a wide range of doses (0.1; 0.25; 1; 4 and 10 Gy) the greatest radioresistance of their phagocytic and lysosomal activity is shown. Gamma-rays at low doses (0.1 Gy) stimulated the formation of extracellular traps by neutrophils in vitro using activated pyrogen (Pyrogernalum). The article presents some mechanisms of RAR in vivo. The study of radioadaptive response should be considered as an indispensable factor for assessing and controlling individual human radiosensitivity.