21 
In the methods used before this time (lS9d), when the toxine and 
antitoxin were injected separately into the guinea pig the substances 
first met each other in the body of the animal. The time it takes for 
the toxine to meet and neutralize the antitoxin when these substances 
are injected separately into an animal naturally varies with the indi- 
vidual conditions of absorption, and exact results therefore were not 
possible. It was found, on the other hand, that if the two substances 
are mixed in a test tube and the mixture is injected into guinea pigs, 
constant and uniform results are obtained. 
At this time (1891) an old test toxine. obtained by Behring and pre- 
served with 0.5 per cent phenol, was used as a basis for measuring 
the strength of the antitoxic serums. Ten times the minimal fatal 
dose of this toxine for a guinea pig weighing 200 to 300 grams was 
mixed with varying quantities of serum the strength of which was to 
be determined. The mixture was at once inoculatf d into guinea pigs 
and the animals closely watched for local reaction at the site of inocu- 
lation. for loss of weight or other symptoms. As the symptoms appear 
in a few days it was possible, in carrying out the tests according to 
this method, to determine the strength of an antitoxin in a short time. 
This is Behring's method of measuring the strength of diphtheria 
antitoxic serum. 
The strength of the serum was determined in accordance with the 
tests carried out by Behring's method, as that quantity of serum which 
completely neutralized the ten minimal fatal doses. The toxine was 
considered to be completely neutralized if the guinea pig showed 
neither local nor constitutional effects of any kind. At that time (1894) 
Ehrlich considered the method as outlined above to be trustworthy, 
and established an axtitoxix uxit as that quantity of serum which 
required just 1 c. c. to neutralize ten times the minimal fatal dose of 
the diphtheria poison. The antitoxic serum, which had precisely this 
power, he considered as a normal serum. 
Later von Behring and Ehrlich found stronger serums and modified 
the unit, which they now call the immunity unit, to be that quantity 
of antitoxin, viz, antidiphtheritic serum, which will neutralize 100 X 
MED for a guinea pig weighing 250 grams. Ehrlich's method for 
car lying out the tests in accordance with this new conception was as 
follows: 
Ten times the minimal fatal dose is mixed with a definite quantity of 
serum and this mixture injected subcutaneously into a guinea pig. If 
the guinea pig lives the quantity of the serum used contains at least 
one-tenth of an immunity unit. 
For example, if the minimal fatal dose of toxine is 0.005. ten times 
this quantity, or O.05, represents ten times the minimal lethal dose 
(lOxMLD). 
