DIPHTHERIA ANTITOXIN. 413 



proportions, and a series of guinea-pigs are injected with 

 mixtures of 100 times the minimum fatal dose oi the 

 toxin and varying quantities of the diluted antitoxic 

 serum. For this purpose guinea-pigs of approximately 

 250 grammes 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, sur- 

 vives 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 centimetre. Antitoxic serums are frequently 

 of such high degree of potency that they contain from 

 800 to 1000 immunity units in each cubic centimetre. 



