7 
The American unit for measuring the strength of tetanus antitoxin 
may be defined as the neutralizing power possessed by an arbitrary 
quantity of antitetanic serum preserved under special conditions to 
prevent deterioration in the Hygienic Laboratory of the Public 
Health and Marine-Hospital Service. This arbitrary quantity now 
contains ten times the amount of tetanus antitoxin necessary to 
neutralize somewhat less than 100 minimal lethal doses of a standard 
toxine for a 350-gram guinea pig— that is, one-tenth of a unit mixed 
with 100 minimal lethal doses of the standard toxine contains just 
enough free poison in the mixture to kill the guinea pig in four days 
after subcutaneous injection. 
The standardization of tetanus antitoxin does not differ radically 
from the standardization of diphtheria antitoxin. The toxines and 
antitoxins are measured against each other reciprocally, so that 
change or deterioration of either the standard toxine or the standard 
antitoxin rnay readily be determined. Duplicate toxines and anti- 
toxins made from time to time will act as checks against deteriora- 
tion of either the standard toxines or antitoxins. 
The value of an unknown serum is measured indirectly through 
the toxine, using the L+dose as the test dose. The L+dose is 
the smallest quantity of tetanus toxine that will neutralize one-tenth 
of an immunity unit, plus a quantity of toxine sufficient to kill the 
animal in just four days. 
The toxine and not the antitoxin is given out to licensed manufac- 
turers and others interested for the purposes of standardizing their 
serums. The, L+ or test dose of the particular toxine (A) now 
dispensed contains just 100 minimal lethal doses for a 350-gram 
guinea pig. This particular toxine is very stable and has not changed 
appreciably in two years. As soon as it alters or is exhausted the 
next toxine that will be issued may contain considerably more or less 
than 100 minimal lethal doses, but the test dose will contain precisely 
the same neutralizing power. 
The antitoxic serum, for the purposes of this standard, was obtained 
from a single horse. The serum was reduced to dryness and ground 
to an impalpable powder. The powder so obtained is preserved in 
many vacuum tubes under the influence of pentaphosphoric acid. 
These tubes are kept in absolute darkness at a constant temperature 
of 5° C. Every two months, or oftener, one of these tubes is opened, 
dissolved in a solution of glycerin 66 parts and isotonic salt solution 
34 parts, and tested. 
While the tetanus antitoxin preserved in dry powdered form under 
the conditions above named is stable, duplicates of the antitoxins 
made from time to time so as to guard against loss or change will 
insure the permanence of the standard which has now been estab- 
lished. 
