Potencies of tetanus toxoid of combined vaccines (PDT) were determined in guinea pigs by two different methods, namely, “antitoxin titration” method and “toxin challenge” method. The potencies of tetanus toxoids were expressed as the relative potency to the National Standard Tetanus Toxoid (fluid) containing no adjuvant. The potencies determined by the two different methods agreed very well, although ranges of antigen dilution differed considerably depending on the method employed, high dilutions being used in the toxin challenge method. It seemed that the high dilution of combined vaccine (such as 1: 360) did not result in the reduction of the adjuvant effect of pertussis vaccine. It was suggested that the adjuvanttoxoid complex might be easily formed by transient contact of both components.
