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't 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 Rehring-Ehrlich definition of an antitoxin unit. 
The results of tests by different experimenters with 
