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1. Considerable differences in antigenic efficiency are encountered among batches of (1) diphtheria toxoid, and (2) Cl. welchii (perfringens) toxins prepared under standardized conditions. These differences were revealed in the results of hyperimmunization of horses.
2. Such differences in antigenic efficiency in batches of diphtheria toxoid were also detected by means of injections into guinea-pigs. Two injections, each of 2 Lf doses, were given at an interval of 28 days, and the response measured by means of antitoxin titrations on bleedings taken 10 days later.
3. A general correlation was found between antigenic efficiency as measured by these two methods.
4. A method is described for comparing the affinity for antitoxin, of batches of diphtheria toxoid, by determining the test dose of mixtures containing 9 Lf doses of toxoid and 1 Lr dose of a fixed toxin. The greater the avidity of the antitoxin, the more toxoid entered into combination with it.
5. A correlation was found to exist between affinity for antitoxin and antigenic efficiency of toxoid, as measured both by the response of horses to hyperimmunization and the response of guinea-pigs to two spaced injections.
6. It is tentatively suggested that crude toxoids may contain molecules possessing varying degrees of affinity for antitoxin, due to slight steric structural differences.
