142 LABORATORY COURSE IN SERUM STUDY 



with such an antitoxin unit will just kill a guinea pig of 250 

 grams in from four to five days, i.e. which mixed with one 

 standard antitoxin unit will give the effects of one free toxin unit. 

 This amount of toxin is known as the L + dose, and of course 

 if such an L + dose of a toxin measured against the standard unit 

 is now mixed with varying quantities of an unknown antitoxin 

 until a mixture of similar effect is obtained, the particular quan- 

 tity of antitoxin used in this mixture will be equal to the original 

 standard antitoxin unit. Thus the antitoxin unit established by 

 Ehrlich is now the standard of measurement for other antitoxins 

 rather than the toxin itself. 



Such standard antitoxin units are preserved in the way 

 indicated above for America at Washington, for Germany in 

 Frankfort, for France at Paris, etc., and the L-h dose of toxin 

 solutions can be determined against these standards and with 

 such L+ quantities of any given toxin new antitoxin can be 

 standardized and portions of these in turn preserved. Con- 

 stancy of measurement is thereby assured if this system is kept 

 under proper governmental supervision without a break in con- 

 tinuity, and without accident. This is practically insured 

 against by the large number of laboratories in the world in which 

 this standard is being kept. 



It is obvious therefore that in the investigation of antitoxic 

 strength three standard units are used. They may be defined 

 as follows : 



The Minimal Lethal Dose (MLD or T) is that amount of 

 toxin which when subcutaneously injected invariably causes the 

 death of a 250-gram guinea pig in from four to five days. 



Limes "Zero" Dose (L ) is that amount of toxin which is com- 

 pletely neutralized by one antitoxin unit so that no trace of reac- 

 tion, local or otherwise, ensues when it is injected mixed with 

 one unit of antitoxin. 



Limes " Todt " Dose (L +) is that amount of toxin which when 

 mixed with one unit of antitoxin and injected subcutaneously 

 will cause the death of a 250-gram guinea pig in four to five days. 



The quantitative relationship between the MLD, the L dose 



