366 BACTERIOLOGY. 



the cases. With the serum from some horses the rashes 

 are very infrequent, while with that from others they 

 occur more often. The same horse will at one time 

 furnish a serum which produces no rashes and at an- 

 other one which gives a great number. No way has 

 yet been found to eliminate them entirely. Filtering 

 and moderate heating produce little effect. Standing 

 for some months causes a precipitate to occur, and the 

 clear serum seems somewhat less liable to produce 

 rashes than when it was fresh. 



The Persistence of Antitoxin in the Blood. When in- 

 jections of toxin are stopped in a horse the antitoxin is 

 slowly eliminated, so that there is a loss of about 20 

 per cent, a week. In from three to five months all 

 appreciable antitoxin has been eliminated. Immunity 

 in human beings lasts from two to six weeks after an 

 injection of 500 units of antitoxin. 



Technical Points upon the Testing of Diphtheria Anti- 

 toxin and the Relations between the Toxicity and Neu- 

 tralizing Value of Diphtheria Toxin. Until within a 

 fairly recent time the filtered or sterilized bouillon in 

 which the diphtheria bacillus had grown and produced 

 its " toxin" was supposed to require for its neutraliza- 

 tion an amount of antitoxin directly proportional to its 

 toxicity as tested in guinea-pigs. Thus, if from one 

 bouillon culture ten fatal doses of " toxin" were re- 

 quired to neutralize a certain quantity of antitoxin, it 

 was believed that ten fatal doses from every culture, 

 without regard to the way in which it had been pro- 

 duced or preserved, would also neutralize the same 

 amount of antitoxin. Upon this belief was founded 

 the Behring-Ehrlich definition of an antitoxin unit. 



The results of tests by different experimenters with 



