In the first decades of the twentieth century, a heterogeneous assortment of groups and individuals articulated scientific, political, and philosophical objections to vaccination. They engaged in an ongoing battle for public opinion with medical and scientific elites, who responded with their own counterpropaganda. These ideological struggles reflected fear that scientific advances were being put to coercive uses and that institutions of the state and civil society were increasingly expanding into previously private realms of decision making, especially child rearing. This essay analyzes the motivations and tactics of antivaccination activists and situates their actions within the scientific and social climate of the Progressive Era and the 1920s. Their actions reveal how citizens of varied ideological persuasions, activists and nonactivists alike, viewed scientific knowledge during a period of swift and unsettling change, when the application of biologic products seemed to hold peril as well as promise.