Focal ablation of erythropoiesis in the bone marrows of five patients who had received radiotherapy for malignant lymphoma was demonstrated by erythropoietic marrow scanning with radioactive iron (59 Fe). Concomitant clinical, morphologic, and ferrokinetic studies confirmed the findings obtained from whole-body linear profiles and erythropoietic marrow scans. These techniques permitted distinction between focal and generalized hemopoietic insufficiency and should prove useful in evaluating the distribution and function of erythropoietic tissue. Results of these preliminary studies suggest that doses of irradiation commonly employed in the initial treatment of malignant lymphoma may prevent repopulation of bone marrow in radiated areas for at least one to three years. Compensatory hyperplasia of hemopoietic tissue occurred in the ribs and axial skeleton of several adult patients.