In the presence of specific antigen 62% of the homologous anti-protein antibodies are rapidly removed from the circulation within one hour after injection and 86 per cent within 6 hours. Once removed from the blood, the antibodies are promptly catabolized, between one third and one half within 6 hours and virtually all between 24 and 48 hours. In the rabbit the rates of catabolism of anti protein antibodies and heterologous serum protein antigens after their elimination from the circulation as specific complexes are comparable.