THE DIPHTHERIA BACILLUS 119 



swelling may not be apparent in the first twenty-four 

 hours, but a rapid fall in weight will at that time fre- 

 quently indicate its probable occurrence within the next 

 twenty-four hours. In this connection it should be borne 

 in mind that guinea-pigs, taken from stock and put into 

 small cages, usually rise in weight when not injured by 

 the action of the toxin. 



Roux's method defines the proportion of serum in 

 relation to the weight of the animal which would protect 

 a guinea-pig against a lethal dose of toxin. 



Ehrlich's method of standardisation, now universally 

 adopted, eliminates errors due to toxoids and toxones. 

 A standard antitoxin is employed, as this is more stable 

 than toxin if dried and stored in vacuo. For use, this is 

 diluted so that one unit is contained in 1 c.c. The 

 laboratory toxin is then standardised with this standard 

 antitoxin, and the exact amount of the toxin is ascertained, 

 which, when mixed with one unit of antitoxin, just 

 suffices to cause the death on the fourth or fifth day of a 

 250-gramme guinea-pig. This amount of the toxin is 

 termed the L+ dose (L = limes = boundary i.e., be- 

 tween life and death, the neutral point L+ meaning 

 that there is one lethal guinea-pig dose of the toxin left 

 unneutralised by the unit of antitoxin). The L+ dose 

 of the laboratory toxin having been ascertained, this 

 amount of the toxin is mixed with varying amounts of 

 the antitoxic serum to be tested, and each mixture is 

 injected into a 250-gramme guinea-pig. The amount 

 of the serum which just suffices to completely protect 

 the guinea-pig from the toxic effects of the L+ dose of 

 toxin is then known to contain one unit of antitoxin. The 

 standardised toxin is preserved by the addition of toluol, 

 and is kept in a cool, dark place; but even then its toxicity 

 gradually diminishes, and it has to be restandardised 

 every three to six weeks. 



As Sudmersen and Glenny found that the active 

 immunity of the doe guinea-pig is transferred passively to 

 her offspring, animals from parents used previously in 

 the test cannot be used. 



The unit of antitoxin corresponds to 105 to 115 minimal 

 lethal doses of a toxin for the guinea-pig, or, roughly, to 

 100 minimal lethal doses. 



The most suitable place for injection of antitoxic 



