DIPHTHERIA 219 



in the course of four or five days. In other words, the single lethal 

 dose constitutes the unit. 



A toxin broth which contains 100 units per c.c., v. Behring has 

 termed a normal toxin solution, and he designates this by the formula 

 DTN, M 250 , which signifies: diphtheria toxin, single normal, in 

 reference to a guinea-pig weighing 250 grams. Of this 1 c.c. would 

 suffice to kill one hundred guinea-pigs and 0.01 c.c. a single animal. 

 A double normal toxin solution, DTN 2 M 25 o would accordingly 

 be one of which one-half that dose, i. e., 0.005 c.c., would suffice to 

 kill a guinea-pig of standard weight. A unit of antitoxin, on the 

 other hand, is that quantity which is capable of neutralizing 100 

 units of toxin, and we designate as normal serum one of which 1 c.c. 

 will neutralize 1 c.c. of normal toxin solution, i. e., 100 units of toxin. 



1 c.c. normal antitoxin serum = 1 c.c. normal toxin solution (suffi- 

 cient to protect 100 guinea-pigs, each against a single lethal dose 

 0.01 c.c. of normal toxin). 



Ehrlich designates as Lf (limes = limit) that quantity of toxin which 

 when mixed with one unit of antitoxin and injected subcutaneously 

 into a guinea-pig weighing 250 grams will kill the animal in four or 

 five days, while he denotes the quantity which is just neutralized 

 by 1 unit of antitoxin, and which will hence not kill the animal 

 when injected together with the toxin as L . 



To determine the strength of a given serum it is for practical 

 purposes only necessary to inject a series of guinea-pigs subcutane- 

 ously, each with a mixture containing, say 100 units of toxin and 

 varying quantities of the serum under consideration. If the animal 

 dies within the first four days the amount of antitoxin was evidently 

 not sufficient to neutralize all the toxin, and the serum hence had a 

 titer lower than 1 unit to the c.c. If death takes place on the fifth 

 or sixth day the antitoxin content is just a unit, and if the animal 

 does not die at all, it must have been stronger than this. Supposing 

 that 1 c.c. of the serum in a dilution of 1 : 1000 had been sufficient 

 to delay death until the fifth or sixth day, then 0.001 c.c. of the 

 concentrated serum would represent one antitoxin unit, and its 

 actual titer would hence be 1000 units per c.c. 



In Germany the production of antitoxin is carefully supervised 

 by the government and every preparation tested in the Institute 

 for Experimental Therapy, of which Ehrlich is the head. In the 

 United States there are now also stringent laws regulating its 



