Labeled lymphoid cells obtained from the lymph nodes draining a skin graft were administered to syngeneic mice of the opposite sex. Labeling of proliferating cells in the lymph nodes was accomplished by the repeated injection of tritiated thymidine into the base of the graft. Labeled cells, presumably donor lymph node cells, were present in small number evenly distributed in autoradiographs of sections of the thymus of both neonatal and adult recipients. At the time of observation, 1, 2, or 3 days after donor cell inoculation, donor cells were not observed in mitosis. The exemption of the thymus from direct involvement in the immune response cannot be accounted for by the exclusion of lymph node cells and therefore it is more likely to depend on factors operating within the thymus itself.