UNIT OF ANTITOXIN 339 



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 cer- 

 tain 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 105120 minimal lethal 

 doses of a toxin broth, the extremes which Ehrlich 

 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 thera- 

 peutic use. The toxin is preserved by the addition of 

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

 restandardised every few weeks. 



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

 dardised 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, 0-04, 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 antitoxin 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 must be kept as constant as possible, guinea- 

 pigs weighing 250 grm. or thereabouts employed, and to 

 eliminate irregularities a number of animals must be used. 



222 



