280 A MANUAL OF BACTERIOLOGY 



perature of 65 C. for an hour and filtering before injection, 

 much, larger initial doses can be given and some degree of 

 immunisation attained, and subsequently the ordinary 

 broth cultures may be injected in large doses. Individual 

 horses vary much in their capacity to yield antitoxin : on 

 the whole those that are moderately sensitive to the 

 toxin seem to produce most antitoxin ; a horse to be of 

 value should after three months' treatment 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 coagulate, 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 multiplication of any 

 stray germs that may have gained access during the 

 various manipulations. 



Standardisation of antitoxin. The potency of diphtheria 

 antitoxin is always described in " units " and is estimated 

 by ascertaining the quantity of antitoxin required just 

 to neutralise a certain amount of a standardised toxin 

 when both are injected into a 250-grm. guinea-pig. For- 

 merly, 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 antitoxin 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 anti- 

 toxin 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 



