DIPHTHERIA ANTITOXIN. 425 



2. Ehrlich's Method. Ehrlich has recently introduced 

 the use of a standard diphtheria antitoxin in a dry state 

 which contains 1700 immunizing units in each gramme. 

 This standard antitoxin is distributed by the Institute 

 for testing serum at Frankfort-on-the-Main, and is now 

 being used in a great many places for the standardiza- 

 tion of diphtheria antitoxin. A test toxin is prepared, 

 corresponding to this standard antitoxin, and with this 

 toxin the strength of the unknown serum is titrated. 



If, for instance, the test toxin is of such a strength 

 that 0.003 c.c. represents the minimum fatal dose for a 

 guinea-pig of 250 grammes, then 0.3 c.c. would represent 

 100 times the minimum fatal dose of toxin, and, accord- 

 ing to Ehrlich's standard, an immunity unit is that 

 amount of antitoxic serum which will neutralize 100 

 times the minimum fatal dose of toxin. In performing 

 the test to estimate the strength of an antitoxic serum, 

 the antitoxin is diluted with sterile water in varying 

 proportions, and a series of 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 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. 



