BACTERIUM XEROSIS 507 



guinea-pigs are injected with mixtures of 100 times the 

 minimum fatal dose of the toxin and varying quantities of 

 the diluted antitoxic serum. For this purpose guinea-pigs 

 of approximately 250 grams weight are employed. If, for 

 instance, a guinea-pig receiving 100 times the minimum fatal 

 dose of toxin, and 0.1 c.c. of the diluted antitoxic serum, 

 survives for four days, then 0.1 c.c. of the serum represents 

 an immunity unit of antitoxin. 



An antitoxic serum of this strength, therefore, contains 

 10 times the normal amount of antitoxin, because it con- 

 tains the immunity unit in only 0.1 c.c.; a normal anti- 

 toxin being one in which an immunity unit is contained in 

 one cubic centimeter. Antitoxic serums are frequently of 

 such high degree of potency that they contain from 800 

 to 1000 immunity units in each cubic centimeter. 



