A consideration of some methods by which the cost of potency assays for diphtheria and tetanus vaccines might be reduced.

Although recommended by WHO the high cost of parallel line quantal response assays for diphtheria and tetanus vaccines has retarded the introduction of comparative assays for these vaccines in developed countries and for the same reason is likely to discourage the introduction of adequate control standards in developing countries. These costs are mainly due to the large numbers of animals needed to obtain adequate precision when responses are assessed simply in terms of death or survival. The use of intradermal challenge scores for diphtheria and of semiquantal scores based on the onset of symptoms for tetanus together with the prospects for use of the same animals for both assays are discussed. Data are presented to show that the adoption of such methods would increase the information available from each animal and so reduce the number of animals required for the satisfactory standardization of diphtheria and tetanus vaccines.