Summary and Conclusions: Titrations by means of rabbit precipitin indicate that horse proteins injected intravenously into normal dogs are retained apparently quantitatively in the circulating blood for at least four days. Nevertheless, transfusions of these bloods into partially exsanguinated hypersensitive recipients show that by the end of four days the circulating horse proteins are so altered that they are no longer capable of calling forth recognizable anaphylactic reactions in horse serum hypersensitive tissues. In horse serum hypersensitive donors a similar complete anaphylactic detoxication of horse proteins without demonstrable quantitative reduction takes place in from ten to eighteen hours. These findings are in harmony with our previous conclusion that marked chemical alterations take place in specific antigens when injected into the animal body and that many of the essential immunologic adaptations are to the resulting denaturization products (secondary antigens) rather than to the primary antigens originally injected. If this conception of immunological adaptation is correct, it would indicate that there are numerous immunological processes and presumably numerous specific antibodies that can not be demonstrated by current serological methods.