The use of serum to treat diphtheria, a deadly childhood disease, was a ground-breaking therapy in many respects. Its production involved inducing immunity to a specific pathogen in horses (they initially used a culture of the bacteria responsible for diphtheria to induce this immunity, and later replaced this culture with the purified toxin produced by the bacteria) and using the serum separated from the animal's blood to treat human victims of the disease. While it can be viewed as an extension of the techniques used for vaccination – the manipulation of a living organism (specifically its immune system) to produce a medicament for use in another species – its status as a treatment for a widespread and deadly disease put it in a class of its own. Furthermore, serum production, starting in 1894, required microbiological techniques and savoir-faire that were beyond the reach of the majority of pharmacists and medical doctors at that time. This is a key reason why, unlike the majority of medicines that were still prepared in the pharmacist's laboratory according to directions provided by the pharmacopoeia, the serum was produced in relatively large quantities at a small number of specialized facilities.

One particularly challenging element in the serum production process was the evaluation of its therapeutic or immunization potential.