BACTERIUM XEROSIS 479 



normal toxin is mixed with increasing quantities of the 

 serum, and these mixtures are injected subcutaneously into 

 guinea-pigs; the quantity of the serum which suffices to 

 neutrahze that amount of normal toxin — i. e., that keeps the 

 animal alive for four days or longer — contains one immunizing 

 unit. 



2. Ehrlich's Method. — Ehrlich introduced the use of a 

 standard diphtheria antitoxin in a dry state which contains 

 1700 immunizing units in each gram. This standard anti- 

 toxin, distributed by the Institute for testing serum at 

 Frankfort-on-the-Main, is now being used in a great many 

 places for the standardization of diphtheria antitoxin. A 

 test toxin is prepared, corresponding to this standard anti- 

 toxin, 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 grams, then 0.3 c.c. would represent 100 times 

 the minimum fatal dose of toxin, and, according to Ehrlich's 

 standard, an immunity unit is that amoimt 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 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. 



