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Autoradiographic investigations with the use of 3H-thymidine were performed on 140 noninbred albino rats. The fate of the label in the cells of the myeloid series was followed in the experimental and control groups. The animals of the experimental group were treated with carminomycin in the LD50 30 minutes after injecion of 3H-thymidine. The mitotic cycle of the nondifferentiated cells and promyelocytes + myelocytes in the control animals was equal to 24 hours, the period and rate of the metamyelocyte maturation being 10 hours and 0.81 per cent/hour respectively. The mitotic cycle of the nondifferentiated cells and promyelocytes + myelocytes during the first division after carminomycin administration increased up to 48 hours due to the block of G2-period, the period and rate of the metamyelocyte maturation being 18 hours and 0.43 per cent/hour respectively. The myelotoxic effect of carminomycin was due to the damage of the dividing cells in G2-period of the mitotic cycle, decreased rate of differentiation and death of the cells after or during the first and second divisions.