HISTORIC DEVELOPMENT OF IMMUNOLOGY 71 



titoxin mixtures. The standardization is com- 

 plicated because of the fact that filtered cul- 

 tures contain substances known as toxoids, 

 which are non-poisonous but which have the 

 power of neutralizing antitoxin. Hence, the 

 ratio between the lethal dose of a toxin and its 

 power to neutralize an antitoxin is not con- 

 stant. The antitoxin, however, is more stable 

 and uniform in its composition and is used as 

 the standard. In this country, all diphtheria 

 toxin used for testing the potency of Diph- 

 theria Antitoxin is tested by titration with a 

 standard antitoxin furnished by the Hygienic 

 Laboratory of the Public Health Service of 

 the United States Government in Washington. 

 Unit The unit of Diphtheria Antitoxin was 

 originally defined as the amount which will just 

 neutralize one hundred fatal doses of toxin for 

 a 250-gram guinea-pig. To determine the unit 

 strength of an unknown antitoxin, a standard 

 dose of toxin must be used which, when mixed 

 with the official standard unit of antitoxin, will 

 cause death of a 250-gram guinea-pig in 96 

 hours. To determine the standard dose of 

 toxin, various amounts of toxin are mixed with 

 one unit of the standard antitoxin and injected 



