Suppression of the mixed lymphocyte reaction CMLR) by a soluble factor produced by alloantigen-activated spleen cells requires genetic homology between the factor-producing cells and responder cells in MLR. The ability of lymphocytes used as MLR responder cells to adsorb MLR suppressor factor was tested to investigate the expression of a receptor structure for suppressor molecules. Normal spleen or thymus cells had no effect on suppressor activity. Concanavalin A (Con A)-activated thymocytes, however, effectively removed suppressor activity, suggesting that the receptor is expressed only after activation and is not present or not functional on resting cells. Significantly neither phytohemagglutinin- nor lipopolysaccharide-activated lymphoid cells adsorbed the factor. Furthermore, only Con A-activated thymocytes demonstrating genetic homology with the cell producing suppressor factor for H-2 regions to the right of I-E were effective adsorbants. Alloantigen-stimulated spleen cells syngeneic to the suppressor cell also removed suppressor activity. These data support an hypothesis that subsequent to stimulation in MLR, T lymphocytes express a receptor, either through synthesis or alteration of an existing molecular structure, which then provides the appropriate site for interaction with suppressor molecules. © 1976, Rockefeller University Press., All rights reserved.