Charles Maclean is generally thought to have played an important role in the contagion debates of the early nineteenth century and to have prompted two parliamentary inquiries into the issue. The author examines the effects of Maclean's efforts to relocate the contagion debates from the medical to the public sphere. The author shows that Maclean's tactics challenged the exclusivity of medical knowledge by ceding power to decide the debate to a non-medically expert Parliament. The author also demonstrates how this conflict laid bare the side-by-side existence of two probative systems in medical debates during the early nineteenth century by examining the medical profession's struggle to establish what type of evidence would be considered probative and what type of witness would be considered competent to give it.