Serotherapy for the treatment of diphtheria represented a major therapeutic innovation at the end of the nineteenth century. The manner in which large-scale production of this medicament was undertaken and the regulations that governed its production and distribution were important elements of public health policy in France as in other European countries. This paper describes the dominance of the Pasteur Institute in this field and, starting from this observation, explores what this event in the history of medicine can tell us about the governance of public health in fin-de-siècle France. The particular organization of this institute and its monopoly of specialist microbiological knowledge allowed it to raise money for serum production from both private and public sources, walking the line between a commercial pharmaceutical venture and a philanthropic enterprise.