Carrier-primed T lymphocytes can suppress high avidity IgG anti-hapten antibody secretion within 90 min. in vitro if the suppressor and target cells are primed with the same carrier determinants. Suppression seems to be directed to the antibody secreting cell since the effect is rapid and does not depend on macrophages or T cells in the target cell population. Suppression can be blocked by inclusion of soluble carrier in the cell mixture or by treatment of the target cells with anti-carrier antibody or pronase. Moreover, suppression can be augmented by PFC exposure to the soluble hapten-carrier conjugate. Finally, carrier specificity may be altered by preincubation of the target population with a hapten-heterologous carrier before addition of suppressor cells specific for the heterologous carrier. 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.