To investigate the blood system response to the use of radiation therapy with metronidazole as a radiosensitizer, a study was made of the peripheral blood and bone marrow (at the light and electron microscopy level) in 46 patients with laryngeal cancer who had been subjected to local therapeutic irradiation at a summary focal dose up to 32 Gy. Twenty-eight patients received radiotherapy in the presence of a radiosensitizing effect of metronidazole, the others without a modifier. The patients ranged in age between 30 and 65. It has been shown that the medullary hemopoiesis of patients with laryngeal cancer is characterized by a considerable variability of the main indices: from hyperplasia of the hemopoietic tissue to the depression of myelopoiesis. The general signs of the bone marrow state were the intensity of erythropoiesis, high lymphocyte, monocyte, reticular and plasma cell counts, change of the partial morphograms of granulocytes and erythrokaryocytes that attest to the inhibition of the maturation of myelokaryocytes. The use of metronidazole as a radiosensitizer as distinct from purely radiation effects results in a high leukocyte count at the expense of the proliferating pool of the granulocytic growth of the bone marrow as well as in a larger share of ineffective erythropoiesis with a corresponding decrease of red blood indices. The disturbances revealed do not restrict indications to the use of metronidazole as a radiosensitizer.