Summary: Suspensions containing adult marrow and thymus cells were far more active in producing anti-sheep red cell hemolysins when transferred to irradiated syngeneic recipient mice than can be accounted for by summating the activities of each cell population alone. Such synergistic activity was not present in 1-day-old thymus cells, but was present in 6-day-old thymus cells. Spleen cells 6 days old, however, were inactive. Adult thymus, but not adult marrow, acted in synergism with a small number of adult spleen cells. The efficacy of adult thymus cells was abolished by 500 r (in air) given to donors. Subcutaneous thymus grafts were not active. The order of injection of thymus or marrow cells was not critical. The data confirm the hypothesis of interaction between thymus and marrow cells, although the nature of this interaction is obscure.