DIPHTHERIA BACILLUS. 373 
tested antitoxin is of immense value in insuring a 
uniform, though not necessarily correct, standard among 
the different testing stations and in allowing of com- 
parison between them. 
The old definition of Behring and Ehrlich, that an 
antitoxin unit contains the amount of antitoxin which 
will protect the life of-a guinea-pig from one hundred 
fatal doses of toxin, must be modified so as to be de- 
fined as that amount of antitoxin which will neutral- 
ize one hundred fatal doses of a toxin similar to that 
adopted as the standard—namely, one having the char- 
acteristics of toxins in cultures at the height of their 
toxicity. 
The actual test of an antitoxin serum is, therefore, 
carried out as follows: Six guinea-pigs are injected 
with mixtures of toxin and antitoxin. In each of the 
mixtures there is 100 times the amount of a toxin such 
as just described, which will kill 250 grammes of 
guinea-pig on an average in 96 hours. In each of the 
mixtures the amount of antitoxin varies; for instance, 
No. 1 would contain 0.002 ¢c.c. serum, No. 2, 0.003 
c.c., No. 8, 0.004 c.c., No. 4, 0.005 ¢.c., ete. If, at 
the end of the fourth day, Nos. 1, 2, and 3 were dead, 
and Nos. 4, 5, and 6 were alive, we would consider 
the serum to contain 200 units of antitoxin for each c.c. 
When we mix only ten fatal doses of toxin with one- 
tenth of the amount of antitoxin used with 100 fatal 
doses we usually consider that the guinea-pig must not 
only live but remain well. 
The Relation of Bacteriology to Diagnosis. I believe 
that all experienced clinicians will agree that, when 
left to judge solely by the appearance and symptoms 
of a case, there are certain mild exudative inflamma- 
