<p>The radioprotective efficacy of granulocyte colony-stimulating factor in the form of pegfilgrastim at fractionated irradiation was evaluated on male ICR (CD-1) mice. Mice were exposed to five daily exposures to X-rays at a dose of 2.5 Gy. Three hours after each exposure, pegfilgrastim was injected subcutaneously into mice at a dosage of 0.5 μg/g. The drug was also administered daily 3–7 days after the last irradiation. The efficacy of this therapeutic regimen was evaluated on the basis of the degree of DNA damage in spleen cells 30 min after the last irradiation; hematologic parameters were evaluated after 30 min and 3 days; Schiff bases, trieno, and oxodiene conjugates in the liver were evaluated after 30 min; and thymus and spleen weight, the number of karyocytes in the femur, and the content of thiobarbiturate-reactive products in the liver were evaluated after 3 days, as well as on the basis of 30-day survival. The application of the therapeutic regimen contributed to a significant correction of irradiation-induced oxidative stress: according to the criteria of the content of Schiff bases, trieno and oxodieno conjugates, the group receiving pegfilgrastim corresponded to the intact mice, the content of thiobarbiturate-reactive products in the liver decreased relative to the irradiated control, but the parameters of intact mice were not reached. Pegfilgrastim administration did not compensate for the irradiation-induced bone marrow cell death 3 days after the last irradiation and could eventually ensure the survival of only 3 mice out of 14 individuals receiving the drug. Thus, for effective application of pegfilgrastim during fractionated irradiation at a total dose of 12.5 Gy it is necessary to increase the number of irradiation sessions with decreasing dose of each session, or to decrease the total radiation dose. Joint use of pegfilgrastim with other radioprotective drugs with other mechanisms of action is also promising.</p>