ANTITOXINS 45 



in large flasks in a refrigerator. From this as needed 

 small vials are filled. The vials and their stoppers, as 

 indeed all the utensils used for holding the serum, must be 

 absolutely sterile and every possible precaution must be 

 taken to avoid contamination of the serum. An anti- 

 septic may be added as a preservative, but is not neces- 

 sary. Diphtheria antitoxin, when stored in vials and kept 

 in a cool place, away from light and air, contains within 

 10 per cent of its original strength for, at least, two 

 months, after that it can be used by allowing a maximum 

 deterioration of 3 per cent for each month. 



Testing the Strength of the Antitoxin. For seventeen 

 years the United States Hygienic Laboratory at Wash- 

 ington has prepared a standard antitoxin which is dis- 

 tributed to laboratories throughout the United States and 

 which is to be used in standardizing toxins and antitoxins. 

 In this way the unit is kept constant in all laboratories 

 from year to year. The test to estimate the number of 

 units in i cc. of an unknown serum is, therefore, carried 

 out as follows : 



Six guinea pigs are inoculated with mixtures of tested 

 toxin and varied amounts of the serum to be tested. In 

 each of the mixtures there is just the amount of toxin 

 sufficient to neutralize i unit of the standard serum from 

 Washington. The amount of unknown serum in the 

 mixtures varies, for instance, No. i would contain .002 

 cc. serum; No. 2, .003 cc.; No. 3, .004 cc.; No. 4, .005 cc., 

 etc. If, at the end of the fourth day, Nos. i, 2 and 3 

 were dead and Nos. 4, 5 and 6 were alive, we would con- 

 sider the serum to contain 200 units of antitoxin for each 

 cubic centimeter. The mixed toxin and antitoxin must 

 remain together for fifteen minutes before testing. 



