The effect of tetrachlorodecaoxide (TCDO) treatment after total-body irradiation (TBI) with γ-rays (single dose, about LD 50) on the development of radiation-induced leukemia was tested in rats. TCDO was applied intravenously from day 4 through day 11. The control group was exposed to the same dosage of X-rays (TBI), but received physiological saline solution instead of TCDO. Compared to the control group, TCDO therapy initially markedly increased the survival rate: 72 versus 44% (6 months after TBI) and 36 versus 20% (1-year survival rate). The overall survival, however, was not significantly prolonged. TBI caused leukemia in 36% of the rats in the irradiation control group without TCDO treatment, however, none of 24 rats treated with TCDO after X-ray exposure developed leukemia. Since in this study TCDO was only administered for 8 days during the acute phase of radiation syndrome, we suppose that additional TCDO treatment at various times later on would lead to even better results