Following i.v. immunization of the normal outbred rabbit with sheep (SRBC) or horse (HRBC) erythrocytes, antigen-specific suppressor cells are generated in the thymus capable of inhibiting the generation of haemolytic plaques by the autologous or allogeneic splenic antibody-forming cells (AFC) in the plaque-forming cell (PFC) assay. These suppressor cells secrete an antigen-specific suppressor factor in short-term (4-24 hr) culture in vitro. The suppressor cells are not detected in the thymus prior to Day 4, exhibit peak activity between Days 5 and 11 post-immunization, and decline slowly thereafter. Suppressor cells can no longer be detected in the thymus by Day 60 postimmunization. Suppressor cells are not detected in any of the other lymphoid organs of the immunized rabbit nor in any lymphoid organ in the unimmunized rabbit. The thymic suppressor cell is a T cell with surface receptors for the antigen (SRBC or HRBC) and for FcG. On the other hand, the AFC B cells generated in the spleen of the immunized rabbit possess cell-surface receptors for only the antigen and not for FcG. Both the suppressor cells and the secreted suppressor factor act directly on the AFC B lymphocytes to inhibit the generation of antigen-specific haemolytic plaques in the PFC assay.