140 



VETERINARY BACTERIOLOGY 



placed 1700 U. I. in each tube. Each month a tube is opened 

 and its content of serum dissolved in 1700 c.c. of a mixture of 

 water and glycerin. Each cubic centimeter, therefore, contains one 

 immunity unit of antitoxin. A careful study of the toxin which 

 Ehrlich had used in preparing this standard showed him that it 

 contained, in addition to the 100 M. L. D. of toxin, an equal 

 amount of toxoid. Theoretically, therefore, this immunity unit 

 prepared by him contains sufficient antitoxin to neutralize 200 

 M. L. D. of a pure toxin. In view of the fact that all antitoxin 



cotTon 



dr\ej serum 



Fig. 69. A tube used for the preservation of antitoxin by the Hygienic 

 Laboratory of the Public Health and Marine Hospital Service : a, Serum scales, 

 or antitoxin; 6, phosphorus pentoxid, an active dehydrating agent (M. J. 

 Rosenau, Bulletin No. 21, Hygienic Laboratory). 



in this country is now standardized with reference to this U. I. 

 of Ehrlich, the immunity unit for diphtheria antitoxin has been 

 redefined as an amount of antitoxin equivalent to that contained 

 in 1 c.c. of solution when the contents of the tubes prepared by 

 Ehrlich are dissolved in 1700 c.c. of water. In the United States 

 a similar set of tubes (Fig. 69) has been prepared by the Hygienic 

 Laboratory of the Public Health and Marine Hospital Service, 

 and the standard serum is sent from that laboratory to the manu- 

 facturers. 



The old antitoxin cannot be used directly to determine the 

 potency of new antitoxin, but a lot of toxin must first be standard- 

 ized. It is necessary to express the strength of the toxin in terms 



