Mouse spleen lymphocytes proliferating under the influence of SRBC, or educated thymocytes were labelled in vivo or in vitro with 3H-thymidine and transferred intravenously to syngeneic recipients, which had received a subcutaneous injection of the same, or a non-cross-reacting, antigen (SRBC, rat or chicken RBC, DNP-proteins) into the right front footpads. The left footpads had been injected with a control (non-cross-reacting) antigen. The right (ipsilateral) and left (contralateral) regional lymph nodes were compared on the basis of their radioactivity and radioautography. SRBC-containing lymph nodes retained specifically both viable primed spleen cells and educated thymocytes. Treatment of donor spleen cells with anti-T serum and complement prevented specific retention. Administration of anti-SRBC serum to the recipients had a similar effect. Retention occurred during 6 days after administration of the antigen to the recipients and was mediated by radioresistant host lymph node cells. It declined profoundly when host macrophages were blocked with charcoal particles. The data obtained suggest a major role of the direct contact between T cell receptors and macrophage-bound antigen in the specific lymphocyte retention.