Rabbits were injected intravenously with bovine serum albumin (BSA) and bacteriophage T2 (T2). 2–3 wk later, anti-BSA was removed from such animals by a procedure which involved exposure of removed plasma to an immunoadsorbent (125I-BSA bound to bromoacetyl cellulose) and return of the adsorbed plasma to the animal. This resulted in removal of the majority of antibody activity to BSA without affecting antibody levels to T2. 1–2 days later, anti-BSA levels began to rise, and reached peak levels usually 5 days after the removal of antibody. Antibody levels to T2 did not change. No evidence was obtained that BSA was released from the immunoadsorbent into the circulation of the rabbits. Thus, only trace amounts of radioactivity were released into the plasma; most of the radioactivity was equally coprecipitable with BSA or human gamma globulin and their specific antibodies; the released material was not demonstrated to be immunogenic in primed rabbits; and the released material did not elute with BSA on gel filtration. The results are interpreted as evidence that serum antibody acts as a regulatory mechanism for antibody formation during the conventional antibody response to a metabolizable antigen.