186 INFECTION AND IMMUNITY. 



toxin also is determined, this being the amount 

 which is exactly neutralized by the unit of anti- 

 toxin. The use of the two doses serves to eliminate 

 subjective errors on the part of the observer. The 

 L-f- and LO doses of toxin are then used to de- 

 termine the value of new antitoxins. That quan- 

 tity of the new serum which, when mixed with the 

 L+ dose of toxin, causes the animal to die in 

 four to six days, contains 1 unit of antitoxin. If, 

 for example, 1/100 c.c. accomplishes this result, 

 the serum is of one hundredfold strength, i. e., 1 

 c.c. would contain 100 antitoxic units. 



In accordance with the Act approved July 1, 

 1902, the United States Public Health and Marine- 

 Hospital Service has established a standard unit 

 for this country. The unit is based on that of 

 Ehrlich just described and was made by com- 

 parison with the normal unit obtained from Ehr- 

 lich's Institute, Frankfort a. M., Germany. 



Antitoxins are purchased on the, open markets 

 by officers of the Public Health and Marine- 

 Hospital Service and tested in the Hygienic 

 Laboratory for potency, freedom from contamina- 

 tion by bacteria and chemical poisons, especially 

 tetanus toxin, and finally to insure against excess 

 of preservatives. 



The method of determination of potency is sim- 

 ilar to that used by Ehrlich and previously 

 described. 



White mice are inoculated to test for an excess 

 of preservative. (Trikresol being the one most 

 employed.) If the mouse shows trembling or other 

 symptoms of poisoning after subcutaneous injec- 



