Abstract
The origin of the lymphoid cell populations of the tonsil was studied by sublethal whole body irradiation of normal and thymectomized animals. Two clearly distinguishable cell populations appeared to be present in the tonsil: a thymus-dependent population, situated in the interfollicular areas, and another presumably bone marrow derived lymphoid cell population, localized in the follicular structures. From the experimental data obtained, it is concluded that no fundamental difference exists between the lymphoid tissue of the tonsil and that of the regional lymph nodes in the adult animal.