TESTING OF ANTITOXIN. 479 



The method of determining' the strength of an antitoxic serum 

 can be shown by the following example: 



The minimum fatal dose of the particular toxin employed was 

 found to be 0.02 c.c. Hence, 0.2 c.c. of this toxin, when injected into 

 each guinea-pig, per 250 g. body-weight, represents 10 times the 

 minimum fatal dose. In order to measure out this amount it is advis- 

 able to add 1 c.c. of the toxin to 4 c.c. of the NaCl solution. 1 c.c. of 

 this dilute toxin (A) represents 0.2 c.c. of the original poison. 



1 c.c. of toxin A corresponds to 10 times the minimum fatal dose 

 for a guinea-pig weighing 250 g. It is not always possible to obtain 

 animals that will have exactly the same weight. They should, how- 

 ever, weigh as close to this amount as is possible; and, a correspond- 

 ing correction for such variation should be made when measuring out 

 the toxin and antitoxin. Thus, for an animal weighing 300 g. the dose 

 of toxin A is ascertained from the proportion: 



300 : x : : 250 : 1 x = 1.2 c.c. toxin A. 



The calculated amounts of toxin A should then be measured out 

 into small, sterile Eamarch dishes. Eventually, the calculated quan- 

 tities of serum are addfd and each mixture is then injected into the 

 propfer guinea-pig. 



If it is desired to ascertain whether the antitoxin possesses the 

 strength claimed, the contents of the bottle should be drained into a 

 sterile Esmarch dish. By means of an accurate pipette the volume 

 of the serum can be readily determined. If the bottle is said to con- 

 tain 2000 I. U. and 7 c.c. are present it is evident that each c.c. of the 

 serum should contain at least 286 I. U. 



Suitable dilutions of the serum are now made in order to facili- 

 tate the measuring out of small quantities. Thus, 0.1 c.c. of the 

 serum is added to 9.9 c.c. of the NaCl solution = dilution A. 0.1 c.c. 

 of this dilution represents 0.001 c.c. of the original serum. 



A second dilution may be prepared by adding 0.5 c.c. of dilution 

 A to 4.5 c.c. of the NaCl solution = dilution B. 0.1 c.c. of thi,s dilu- 

 tion represents O.OOOl c.c of the original serum. This amount, 

 according to the definition of an immunity unit, corresponds to 1000 

 I. U. Hence, 



0.25 c.c. of dilution B (0.00025 c.c. serum) represents 400 I. U. 



0.29 c.c. of dilution B (0.00029 c.c. serum) represents 345 I. U. 



0.33 c.c. of dilution B (0.00033 c.c. serum) represents 303 I. U. 



0.35 c.c. of dilution B (0.00035 c.c. serum) represents 286 I. U. 



The above amounts, it will be understood, refer to a guinea-pig weigh- 

 ing 250 g. Corrections for variations from this weight should be 



