The American method of standardizing tetanus antitoxin has been studied and two samples (Toxins 3 and 4 of this paper) have been examined.

It has been found that:

1. Toxin 3 remained stable for two years under conditions which were not absolutely ideal, as the tube containing the supply was repeatedly opened during that time and the air in this neighbourhood, especially in winter, is very moist.

Toxin 4 has so far remained stable for 12 months. This toxin has been distributed in small quantities in tubes which have been exhausted and sealed in the flame.

2. By itself each of these toxins has given results consistent within the error of experiment when used to establish the M.L.D. and also to test a serum. But when the same serum was evaluated by both toxins they gave differing values. This dffference in value appears to be due to

(a) Deterioration in Toxin 3 which raised the L + dose from 00006—000075.

(b) Excess of toxicity in Toxin 4.

Whether this is due to increased susceptibility of the race of animals used for the tests can at present be only a matter of surmise.

3. The American method shows clearly that tetanus antitoxin can be heated at 57°C. for one hour on each of three successive days without loss of antitoxin provided no antiseptic (e.g. phenol) be present. Should such a preservative have been added then the loss on heating amounts to about 12% and occurs during the first hour.

4. Tetanus antitoxin containing phenol appears to be more susceptible to heat than diphtheria antitoxin under the same conditions.

The general conclusion arrived at is that, provided control experiments are carried out from time to time (as in the case of standard diphtheria toxin), this method of standardizing tetanus antitoxin is—as claimed for it—simple, accurate and reliable.