It seemed of interest to study the effect on lymphoid tumor incidence of thymectomy at vary ing intervals after irradiation and to determine whether homologous thymic tissue implanted after irradiation can restore the lymphoma susceptibil

duced lymphoid tumors exhibit a tendency to arise in the thymus gland. Furth and his associ ates (1, 8) found that the incidence of spontaneous lymphomas in a susceptible strain was sharply re duced after thymectomy. Subsequently, Kaplan (4) and Law and Miller (6, 7) reported a similar influence of thymectomy on lyinphoma develop ment under other experimental conditions. Furth (1) and Kaplan (4) attributed this phenomenon simply to removal of the tissue of maximal sus ceptibility. However, Law and Miller (6, 7) re ported that the incidence of lymphoid tumors could be restored to essentially normal levels in thymectomized mice by autologous or homologous subcutaneous thymic implants, suggesting that the thymus may be involved in the pathogenesis of the disease in an additional role, possibly of endo cnine nature. In all these investigations, thymectomy was performed before exposure to the tumor-inciting agent, or, in the spontaneous lymphoma studies, at a time when the endogenous agents were pre sumably still active. Moreover, Law and Miller implanted their thymic grafts prior to the start of carcinogen application ; thymic tissue was there fore present in the animal during exposure. It seemed of interest to study the effect on lymphoid tumor incidence of thymectomy at vary ing intervals after irradiation and to determine whether homologous thymic tissue implanted after irradiation can restore the lymphoma susceptibil ity of thymectomized mice. This paper is con cerned with two such experiments.