284 A MANUAL OF BACTERIOLOGY 



would be equal to the minimal lethal dose. But because 

 of the presence of toxone, which also has an affinity for 

 antitoxin, D, the difference between the L+ and the L 

 doses, is usually a multiple (8-12) of the minimal lethal 

 dose. 



From these considerations we are now in a position 

 to define the unit of antitoxin : a " unit " is that amount 

 of antitoxin which will neutralise about 100 minimal lethal 

 doses for the guinea-pig of diphtheria toxin. From certain 

 considerations Ehrlich considers that the unit would 

 exactly neutralise 200 minimal lethal doses of a theoretical 

 toxin, containing only toxin and neither toxoid nor toxone, 

 but, inasmuch as such a toxin is unknown practically, the 

 unit corresponds usually to 105-120 minimal lethal doses 

 of a toxin broth, the extremes which Ehrlich has found 

 being 16 and 136 lethal doses. Having standardised a 

 specimen of toxin by means of standard antitoxin, this 

 standardised toxin is in its turn used to standardise the 

 antitoxic serum which has been prepared for therapeutic 

 use. The toxin is preserved by the addition of toluol, 

 and is kept in a cool, dark place ; it needs to be restand- 

 ardised every few weeks. 



In standardising antitoxin, the L + dose of the stand- 

 ardised toxin is mixed with varying amounts of the 

 antitoxin, the mixtures are injected into guinea-pigs, and 

 the amount of the antitoxic serum which neutralises the 

 L + dose of toxin is thus ascertained. If, for example, it 

 were found that 0'05, OO4, and 0'03 c.c. of the antitoxic 

 serum neutralised the L + dose of toxin, but that the guinea- 

 pig receiving 0'025 c.c. suffered from some local necrosis, 

 wasted, and died in a few days, and the animal receiving 

 0*02 c.c. died in two or three days, 0'03 c.c. of this anti- 

 toxin would be about equivalent to one unit of standard 

 antitoxin, and the antitoxic serum therefore contains 33 

 units per c.c. For all the experiments the conditions 



