DIPHTHERIA ANTITOXIN 335 



yield an anti-toxic serum containing not less than 300 

 units per c.c. The required potency having been attained, 

 as shown by the test described below, the horse is bled 

 with aseptic precautions, the blood is allowed to coagu- 

 late, and the serum is drawn off and filled into sterile 

 bottles each containing a dose of the antitoxic serum. 

 A small amount of antiseptic, such as trikresol, is generally 

 added as a precautionary measure to prevent the multi- 

 plication of any stray germs that may have gained access 

 during the various manipulations. 



Standardisation of antitoxin. The potency of diph- 

 theria antitoxin is always described in " units " and is 

 estimated by ascertaining the quantity of antitoxin 

 required just to neutralise a certain amount of a stan- 

 dardised toxin when both are injected into a 250-grm. 

 guinea-pig. Formerly, by Roux's method, the minimal 

 lethal dose of the toxin is first ascertained, and then the 

 number of grammes of guinea-pig which 1 c.c. of anti- 

 toxin will protect against this minimal lethal dose is 

 determined. If 0-01 c.c. of antitoxin protects a 300-grm. 

 guinea-pig against the minimal lethal dose, 1 c.c. will 

 protect 300 X 100 == 30,000 grm. of guinea-pig, and the 

 immunising value of the antitoxin would be described as 

 30,000. This method is open to the fallacy that if only 

 a portion of the lethal dose be neutralised the guinea-pig 

 may survive, and a fictitious value be given for the 

 potency of the antitoxin. Behring later adopted ten 

 minimal lethal doses as the test dose of toxin, and he 

 termed ten times the amount of antitoxin which protects 

 a guinea-pig against the ten minimal lethal doses a unit 

 (the Behring unit, which therefore 100 minimal lethal 

 doses of toxin), from which the Ehrlich unit, now univer- 

 sally adopted, is derived. Though this method eliminates 

 to a large extent the objections to the Roux method, 

 Ehrlich found that by it the same antitoxin tested 



