THE MANUFACTURE AND USE OF SERA AND VACCINES. 131 



now adopted by all civilized countries is the so-called 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 administered 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. Marine Hospital Service 



FIG. 58. 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. 



at Washington, and every manufacturer of diphtheric 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, deter-- 



