Effects of chronic low-level irradiation on radiosensitivity of mammals (mice) are studied experimentally and by making use of the methods of mathematical modelling. Our own and reference experiments show that chronic low-level short-term and long-term exposures induce, respectively, elevated radiosensitivity and lowered radiosensitivity (radioresistance) in mice. The manifestations of these radiosensitisation and radioprotection effects are, respectively increased and decreased mortality of pre-irradiated specimens (in comparison with previously unexposed ones) after challenge with acute irradiation. The reason of the animals’ death in the experiments was the hematopoietic syndrome among acute radiation syndromes. Therefore the theoretical investigation of the influence of pre-irradiation on radiosensitivity of mice is conducted by making use of the biologically motivated mathematical models which describe the dynamics of hematopoietic system in mice exposed to challenge acute exposure following the chronic one. Modelling results indicate that the radiosensitization effect of chronic low-level short-term (less than 1 month) pre-irradiation on mice is due to increased radiosensitivity of lymphopoietic, granulocytopoietic, and erythropoietic systems accompanied by increased or close to the normal level radiosensitivity of thrombocytopoietic system.