It is widely believed that rabbits sensitized to complex proteins, such as horse serum, cannot be desensitized by the intravenous injection of the specific protein. In order to desensitize an animal, it is apparently necessary to cause the disappearance of the precipitins from the circulation. In the case of purified proteins this disappearance of the precipitins appears to be an established fact (1) and is believed to be due to the union in vivo of the injected precipitinogen with the circulating precipitins. A complex protein, however, consists of a number of antigens, and when injected into an animal, presumably calls forth the production of a corresponding number of precipitins. The removal of the precipitins from the circulation would require that all of the precipitinogens and their corresponding precipitins are in correct proportion for complete precipitation, a condition that is not likely to occur, and as long as precipitins remain in the circulation, complete desensitization is believed to be impossible.