BACTERICIDAL SERUMS. 255 



When the protective power of the serum is de- 

 termined by animal experiment it is not essential 

 to use the serum when fresh; in fact, the native 

 complement in the immune serum may be disre- 

 garded, or, preferably, it may be destroyed by heat. 

 If the latter procedure is adopted, or if an old 

 serum is used in which the complement has de- 

 generated, its reactivation is accomplished through 

 the complement which is present in the body of the 

 experiment animal. There are reasons for believ- 

 ing that a given antiserum requires a particular 

 complement for its reactivation, and that this 

 complement may be present in some animals and 

 absent in others; this will be referred to again. 



To find the value of anticholera serum Pfeiffer 

 prepares dilutions similar to those mentioned 

 above, and to the same quantity of each dilution 

 adds ten fatal doses of a virulent culture of the 

 vibrio of cholera. These are injected into the 

 peritoneal cavities of guinea-pigs and after periods 

 of from forty to sixty minutes hanging-drop prep- 

 arations are made from the peritoneal fluid of each 

 animal to determine the formation of the charac- 

 teristic granules ; the highest dilution which causes 

 this change in the cells stamps the value of the 

 serum. The animal must at the same time be pro- 

 tected against the ten fatal doses of the culture. 



The value of an antityphoid serum may be de- 

 termined in the same way, the result being judged 

 by the protection which is afforded the animal 

 rather than by the formation of granules. 



Antityphoid, antiplague, and some other serums 

 are also tested by injecting the serum twenty-four 

 hours in advance of the culture. 



