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Abstract. In connection with the active space exploration and the search for new sources for tumor radiotherapy, studies of the effects of low doses of radiation, which are characterized by a high LET, are currently of particular interest. The therapy with heavy charged particles becomes of more and more interest all over the world, and many medical centers tend to use heavy ion beams in radiotherapy. We investigated the biological effects induced by accelerated 12 C ions with an energy of 450 MeV/n in the Bragg peak in a dose range of 0.1-1.5 Gy in mice in vivo. It was found that: (1) the dose dependence of the level of cytogenetic damage in the bone marrow is nonlinear; (2) changes of the thymus and spleen weight index depend on the dose and the quality of radiation, and this index is considerably reduced as compared to that of unirradiated mice; and (3) the level of spontaneous ROS production in blood cells increases in comparison with irradiation at the same doses of X-rays. We calculated the ratio of biological effects under the action of accelerated carbon ions to the effects of same doses of X-ray radiation. The obtained coefficient served as an index of the radiation efficiency (IRE). Thus, it was found that the average IRE value for accelerated carbon ions with an energy of 450 MeV/n in the Bragg peak in the dose range examined varied from 1.1 to 2.4 and was independent of selected biological endpoints in mice in vivo.