1. 1. All of 6 purified diphtheria toxins (and toxoids) and 17 antitoxins tested by the semisolid precipitin method demonstrated multiple lines of reaction, the number of lines formed being dependent upon both the antitoxin and toxin used.2. 2. Of the 3 types of antitoxin investigated, low-salt antitoxin generally gave the largest number of lines with any particular toxin, followed by high-salt and enzyme digested material in that order. This difference is thought to be due to the method of preparation of the various types of antitoxins rather than to any inherent difference between the various fractions.3. 3. Antitoxins resulting from short-term immunization tend to show fewer lines with any given toxin or toxoid.4. 4. The number of lines formed by a particular toxin or toxoid with antitoxin appears to be inversely related to the estimated purity of that toxin or toxoid.5. 5. By appropriate absorption techniques, toxin or toxoid can be deprived of its major line of reaction with a particular antitoxin, without markedly altering its pattern of minor lines in the semisolid precipitin reaction.6. 6. The above conclusions strongly support Pope's contention that “purified” diphtheria toxins and toxoids may contain numerous antigens other than toxin and toxoid.7. 7. A crude method of estimating the purity of toxoid preparations by use of the semisolid precipitin reaction is described, and data are presented to demonstrate that values so obtained agree reasonably well with values obtained by Pappenheimer's methods.
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