412 BACTERIOLOGY. 



normal diphtheria toxin 1 c.c. will be sufficient to kill 

 100 guinea-pigs weighing 250 grammes each, or 25,000 

 grammes in weight of guinea-pigs. 



The quantity of antitoxin that is required to just pro- 

 tect 25,000 grammes weight of guinea-pigs from the 

 minimum fatal dose of the toxin is called one immuni- 

 zing unit. If an immune serum contains in 1 c.c. one 

 immunizing unit, it represents a " normal " antitoxin. 



To determine the strength of an immune serum, 1 c.c. 

 of 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 suf- 

 fices to neutralize that amount of normal toxin i. e., 

 that keeps the animal alive for four days or longer 

 contains one immunizing unit. 



2. Ehrlich'a 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 



