Potency assays are designed to estimate quantitatively the contents in a pharmaceutical product of an "effective constituent", and are as such a very special case of the general pharmacological investigation in animals. In the WHO Requirements for adsorbed toxoid vaccines against diphtheria and tetanus the potency assay is used to ensure that the vaccines are biologically active. The potency assay is not designed and not well suited for this purpose. An "activity test", designed to be more like the clinical use of the vaccines, i.e. by using two or three vaccinations, is suggested. By means of this test the vaccines can be shown, in a few animals, to be able to produce the effect vaccines are expected to produce--a strong and lasting immunity.