The assay of diphtheria toxoids in guinea-pigs, with particular reference to the number of injections given.

Contrary to common belief there is a clear correlation between the primary antitoxin response in guinea-pigs and the potential immunity established by the first injection of toxoid. The magnitude of the secondary response is determined by the potency and amount of the toxoid contained in the first dose and is largely independent of both quality and quantity of that contained in the second dose. One-injection and two-injection methods of assay would probably give the same answer if appropriate doses were chosen but the use of more than one injection is recommended because the sera produced are more avid. The first injection could be of graded doses of the toxoids under test and the second the same for all animals, e.g., a suitable dose of a plain toxoid. A response metameter defined as U(0.2) was preferred to log U in analysing the results, U being the antitoxin concentration in international units.