We have been investigating T lymphocyte interactions in the development of effector lymphocytes in a cell-mediated immune response (1–3). We found that mouse or rat initiator lymphocytes, sensitized in vitro against allogeneic fibroblasts, recruit syngeneic effector T lymphocytes. This recruitment reaction can be triggered in the draining popliteal lymph nodes (PLN) of animals by injecting their hind footpads with in vitro sensitized initiator T lymphocytes. The initiator lymphocytes do not themselves proliferate or differentiate into effector cells. Instead, T lymphocytes of the recipient accumulate in the enlarging PLN. These recruited T lymphocytes were found to mediate immunospecific cytotoxicity in vitro, or allograft rejection in vivo, against the same antigens which induced sensitization of the initiator lymphocytes.