492 BACTERIOLOGY. 
human sepsis, which are not very virulent in animals, 
are especially in need of investigation. If those who 
use the serum will send to the laboratories materials 
for cultures this can in time be fully determined. 
The Preparation of the Serum. Antistreptococcus 
serum is obtained from the horse, ass, and sheep after _ 
treatment by repeated injections of living streptococcus 
cultures. The procuring of a serum of the highest 
potency requires a considerable number of animals, for 
some produce with the same treatment a more protec- 
tive serum than others. The serum must be sterile 
from streptococcus as well as from other contaminations. 
The Stability of the Serum. Unfortunately, after sev- 
eral weeks or months, the serum, asa rule, at least, loses 
its protective value. It should be kept in a cold and 
dark place. Not only ourselves, but others, such as 
Aronson, have found this to he true. 
To this deterioration can probably be ascribed the 
failure of Koch, Petruschky and others to find in the 
serum any power to protect animals from infection. 
The Standardization of the Value of the Serum. The 
value of the serum is measured by the amount required 
to protect against a multiple of a fatal dose of a very 
virulent streptococcus. The dose is usually a thousand 
times the average fatal amount of a very virulent 
streptococcus. 
This method gives, as a rule, to those unfamiliar 
with bacteriology an exaggerated idea of the potency 
of the serum. 
A thousand times the amount of a very virulent 
streptococcus culture required to kill an animal by 
producing septicemia is still too little to kill by the 
streptococci injected; it is only their enormous multi- 
plication in the animal which kills. 
