Carrier-specific suppressor T cells can suppress antibody secretion by high avidity IgG plaque-forming cells (PFC) within 90 min in vitro. This process can be blocked by the inclusion of soluble carrier in the cell mixture or by the exposure of target cells to anti-carrier antibodies or pronase. Moreover, suppression can be augmented by PFC exposure to the soluble hapten-carrier conjugate. Finally, carrier specificity may be overcome by preincubation of the target population with a hapten-heterologous carrier before addition of heterologous carrier ATC. Thus, it is likely that high avidity suppression depends upon immunogen bound to the surfaces of antibody-secreting cells which serves as a target for suppressor cells or molecules.