21 
)| In the methods used before this time (1891), when the toxine and 
I j antitoxin were injected separatel}^ into the guinea pig the substances 
! first met each other in the body of the animal. The time it takes for 
I I the toxine to meet and neutralize the antitoxin when these substances 
I I are injected separatel}^ into an animal naturally varies with the indi- 
:,i vidual conditions of absorption, and exact results therefore were not 
I i possible. It was found, on the other hand, that if the two substances 
I are mixed in a test tube and the mixture is injected into guinea pigs, 
' constant and uniform results are obtained. 
I At this time (1891) an old test toxine, obtained b}^ Behring and pre- 
I served with 0.5 per cent phenol, was used as a basis for measuring 
|i the strength of the antitoxic serums. Ten times the minimal fatal 
I dose of this toxine for a guinea pig weighing 200 to 300 grams was 
,1 mixed with vaiying quantities of serum the strength of which was to 
be determined. The mixture w^as at once inoculated into guinea pigs 
and the animals closel}" watched for local reaction at tlie site of inocu- 
, lation, for loss of weight or other S 3 unptoms. As the symptoms appear 
in a few da^^s it was possible, in canning out the tests according to 
; this method, to determine the strength of an antitoxin in a short time. 
Tliis is Behring’s method of measuring the strength of diphtheria 
antitoxic serum. 
I The strength of the serum was determined in accordance with the 
' tests carried out b}" Behring’s method, as that quantity" of serum which 
, complete 1}^ neutralized the ten minimal fatal doses. The toxine was 
' considered to be completel}" neutralized if the guinea pig showed 
neither local nor constitutional effects of any kind. At that time (1891) 
Ehrlich considered the method as outlined above to be trustworth}", 
and established an antitoxin unit 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 precisel}^ 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 
MLD for a guinea pig weighing 250 grams. Ehrlich’s method for 
carrying 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 subcutaneousl}" 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 0.05, represents ten times the minimal lethal dose 
(10 X MLD). 
