After both splenectomized and non-splenectomized mice had received a total amount of five doses of Busulfan at a rate of 0.14 mg/48 h, the recovery of their erythropoietic organs through the addition of 59Fe was investigated, after the cytostatic treatment had been cut off. In the non-splenectomized animals recovery was clear, keeping high 59Fe uptake values, significantly above normal, throughout the 21 days of the experiment. In the splenectomized mice, 59Fe uptake reached a peak of 38 per cent three days after the supply had been cut off, and from then on it descended to subnormal values.