An understanding of the reseed mechanism of lymph nodes is essential to the development of a rationale for radiation therapy in Hodgkin's disease and lymphomas. The fact that major lymphoid compartments above and below the diaphragm can be irradiated without imposing a severe burden on the individual has set the stage for a more aggressive radiotherapeutic attack. This form of treatment has been referred to as “Segmental Sequential Irradiation” (SSI) of the major lymph node bearing areas (1, 2). The defeating factor with this technic is the possibility that, once a segment has been fully irradiated, the malignant lymphoma cell can reseed as irradiation of another segment is in progress. The concept of rapid reseed of lymph nodes after irradiation is well based upon radiobiologic and radiation pathologic studies of normal lymph nodes. Engeset (3) has shown that local irradiation of a lymph node to 3,000 R results in marked degeneration and depletion of lymphocytes in the first day but leads to regeneration of elements by the second day and findings are normal in one week. The barrier function was reduced by 7 per cent at two weeks to 25 per cent at twenty-eight weeks, utilizing human erythrocytes (7 microns), toad erythrocytes (15 microns), and charcoal particles (60 microns). If normal lymphocytes can reseed and grow in lymph nodes, the likelihood of malignant lymphoid and reticular cells doing the same is highly suspect as a mechanism for in-field recurrences after irradiation. The SJL/J mouse Hodgkin's-like reticulum-cell sarcoma has been intensively studied in our laboratory (4). The method of producing a transplantable lymphoma unicentric in origin has been described previously, and earlier experiments have been recorded (1). Experiment IV: Seed and Reseed Mechanism: Reseed as a function of time after irradiation. Method: Briefly, a small inoculum (0.1 to 0.2 cc) is injected into the axillary fold containing 10,000 ± 2,000 cells. The adjacent ipsilateral axillary or neck nodes invariably are the first nodes to appear. This is a constant observation in hundreds of mice so injected. Each study consists of a flight of 50 mice with controls. In this study, 2,000 rads (220 kV, h.v.l. 1.0 mm Cu) was given to a localized right axillary and cervical field sharply demarcated at the midline (Fig. 1). The bolus of cells was injected at one day, one week, one month, and three months after irradiation. There were two separate control groups, due to the interval between animals irradiated up to one month and the three-month groups. Clinical palpation was performed three times weekly to detect nodes. Serial sacrifice at monthly check points was also carried out, and the animals were completely sacrificed six months after the study began.