144 LABORATORY COURSE IN SERUM STUDY 



and the L + dose is one that is by no means regular, an irregularity 

 which has not yet met with an entirely satisfactory explanation 

 and which for the actual practical measurement of antitoxin 

 it is not necessary to go into at present. The student, however, 

 is advised again to refer to the textbooks mentioned above and to 

 read the explanations of this subject carefully. It is easily seen, 

 however, that the definition of an antitoxin unit as it is at present 

 used can no longer be truthfully given as the amount which neu- 

 tralizes 100 fatal doses for guinea pigs of 250 grams, although this 

 is the way it is usually put in textbooks. The original unit 

 measured by Ehrlich and preserved did actually neutralize 100 

 fatal doses of the particular toxin used. However, when the 

 stated amount of this original antitoxin [or of other antitoxins 

 based upon it and similarly preserved] is measured against many 

 other and unknown toxins, the L-f dose of these poisons con- 

 taining amounts of toxoid differing from that in Ehrlich's 

 original toxin will not necessarily contain 100 minimal lethal 

 doses of true toxin. The antitoxin in such mixtures is neutralized 

 not only by the true toxin but also by the toxoid which has 

 different or no poisonous properties, and thus the number of 

 actual minimal lethal doses contained in the mixture may vary 

 from 50 up. An antitoxin unit, therefore, in the modern sense is 

 the amount of antitoxin which when mixed with the L + dose of a 

 standard toxin leads to death of the guinea pig in four to five days. 

 The L + dose of the toxin in this case must have been measured 

 by a previously determined standard antitoxin unit which is the 

 ultimate basis of measurement. 



In order to allow a margin of safety this definition has been 

 further altered of recent years in the following way : 



The antitoxin unit is the amount of antitoxic serum which 

 mixed with the L + dose of a standard toxin will preserve a guinea 

 pig of 250 grams from death. 



PREPARATION OF TOXIN 



Each student is given a 500 c.c. Erlenmeyer flask in which is 100 c.c. 

 of veal infusion broth containing 2 per cent peptone and adjusted to 



