Guinea pigs rendered hypersensitive (delayed-type) to protein antigen can be completely and specifically desensitized by a single injection containing a sufficient amount of the corresponding antigen. Although 1 to 2 mg. of specific antigen are required for complete desensitization, as little as 20 µg. suffices to decrease the size of specific skin reactions in sensitized animals. The duration of non-reactivity lengthens as the amount of antigen in the desensitizing injection is increased, but skin reactivity eventually returns and is accompanied by the appearance of excess circulating antibody. Desensitization can be accomplished with the antigen-antibody complex as well as by "free" antigen. The appearance of delayed skin reactions can be prevented in fully sensitized animals by intravenous desensitization 2 or more hours after intradermal challenge or by simply skin testing with a desensitizing dose of specific antigen. Injection of a desensitizing dose of antigen into specifically sensitized animals also results in a transient anergic state, the implications of which are discussed.