A convenient, practical, and economical test for the determination of the diphtheria-antitoxin content of blood sera is important for a number of reasons, among which may be mentioned the desirability of estimating quantitatively the efficiency of active immunization with toxin-antitoxin, the development of an active immunity after an attack of diphtheria, the antitoxin content of the blood at varying intervals after a single therapeutic or prophylactic injection of diphtheria antitoxin, the effect of one injection of antitoxin on the duration of immunity conferred by a second injection, etc. Such determination of the antitoxin content would help to give us a solid scientific foundation in many experimental and clinical problems connected with the subject of diphtheria. The older subcutaneous test of Ehrlich for determining the antitoxin content by the death of the guinea-pig has been universally accepted as a standard, but it cannot be used with any degree of accuracy when less than 1/20 of a unit of antitoxin to the cubic centimeter of serum is to be determined. The subcutaneous test requires a large amount of serum for each test, especially when the test is made for a fraction of a unit of antitoxin. This amount is not always obtainable in the case of human beings, especially when we are dealing with small children. In addition, the subcutaneous test is expensive in that it requires the use of one guinea-pig for each test. A method, therefore, that would enable us accurately to determine as little as 1/200 unit of antitoxin, require but a small amount of serum, and be saving of animals, would be of considerable advantage. When a larger series of sera is to be tested for small amounts of antitoxin, the intradermal method is practically the only feasible one. Recent work in active immunization against diphtheria' has shown successful results in producing an antitoxic immunity in 90-95% of
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