Thirty-two patients with lymphoma were treated with extended radiotherapy (RT) at a dose of Gy and were studied by ferrokinetic studies and surface counting at various times following irradiation. Loss of hematopoietic activity in the irradiated areas is compensated by increased activity in the non-irradiated areas. Despite the return of peripheral blood counts to normal, the hyperactivity of the non-irradiated bone marrow persists over up to 13 years after RT, while the hematopoietic activity of the irradiated areas remains depressed and is only slightly higher than immediately after RT. The hypoactivity persisted even when the hemopoietic tissues had been subjected to the intense stimulation provoked by an aplasia caused by chemotherapy. However, a recovery was observed for dose of 20 Gy or lower. The hemopoietic activity of the irradiated bone marrow appears to be related to the volume of the marrow irradiated and is higher after a mantle + inverted Y field than after a mantle field. Both marrow scintigraphies with 59Fe in 7 out of 9 patients studied revealed an extension of hematopoiesis into a normally dormant area of the marrow, such as the femora. In 2 patients an erythropoietic activity was observed in spleens which had received a dose of 40 Gy, and extra medullary erythropoiesis was found in approximately two-thirds of the patients.