264 PHARMACEUTICAL BACTERIOLOGY 



Ehrlich unit, which is the amount of serum (antitoxin of the horse) which 

 will just neutralize one hundred times a fatal dose of toxin when adminis- 

 tered to a guinea-pig, weighing approximately 250 grams or one-half pound. 

 The U. S. standard is prepared in the biological laboratories of the U. S. 

 Public Health Service at Washington, and every manufacturer of diph- 



PlG. 64. — Sterilized liter tubes into which the blood drawn from the horse is placed. 

 The top, covered with sterilized cloth, is connected with the canula in the jugular vein 

 of the animal. 



theric antitoxin in the United States is supplied with standard units from 

 this laboratory. The method of procedure is approximately as follows: 

 Eight containers (test-tubes) are set out in a row and numbered or marked 

 serially. Into each tube is poured just one hundred fatal doses of toxin 

 (fatal to a 250 gram guinea-pig, determined experimentally), and a graded 

 amount of the serum to be standardized, so that the first tube has, in all 



