One particularly challenging element in the serum production process was the evaluation of its therapeutic or immunization potential. e active principle of the serum (if, indeed, there was such a thing as an active principle) did not correspond to any known single component, making it di erent from a chemical medicine, the potency (and, in principle at least, e cacy) of which could be evaluated by simply weighing the physiologically active compounds. Determining the serum’s e cacy therefore involved the use of a number of experimental animals and a series of protocols that governed the serum being tested, a sample of standardized diphtheria toxin, and o en a standard serum as well.3