The present study is the first of a series designed to use hypersensitivity of the immediate type to diphtheria toxin or toxoid as a model for the study of the hay-fever type of allergy. The use of the toxin-antitoxin system in the experimental study of allergy possesses several distinct advantages over less well characterized systems such as are encountered in patients sensitive to pollens, grasses, etc. In the first place, diphtheria toxin and toxoid are available in a high state of purity and are reasonably well characterized proteins. Secondly, it is possible to measure by means of the sensitive rabbit intracutaneous test both toxin and antitoxin with considerable accuracy even when they are present in very low concentration. Finally, it has been demonstrated that administration of a single dose of purified diphtheria toxoid into Schick-negative adults is frequently followed by a rapid and high antitoxin response. Quantitative studies (1) have shown that no other antibody is formed in appreciaable amount and therefore the toxin-antitoxin reaction in human beings immunized in this way closely approximates a single antigen-antibody system. Prausnitz and Kustner in 1921 (2) first demonstrated in an allergic individual a serum factor which was capable of causing local sensitivity of the wheal and erythema type in the skin of normal human subjects. (Sensitivity to fish was passively transferred in the original experiments.) Prausnitz and Kustner showed that the reaction was highly specific but they were unable to demonstrate either precipitating or complement-fixing antibodies in the patient's serum, nor was it capable of passively sensitizing guinea pigs to anaphylactic shock. Moreover, they were unable to demonstrate by means of skin tests neutralization of the allergen in extracts of fish by the patient's serum. Most of the original observations were subsequently confirmed and extended to include passive transfer of sensitivity to a variety of antigenic materials.