Research physicians face intractable dilemmas when they consider introducing new medical procedures. Innovations carry the promise of preventing or curing life-threatening diseases, but they can also lead to injury or even death. How have clinical scientists made high-stakes decisions about undertaking human tests of new medical treatments? This book explores this issue as it examines vaccine trials in America during the early and mid-twentieth century. Today's scientists follow federal guidelines for research on human subjects developed during the 1960s and 1970s. But long before these government regulations, medical investigators observed informal rules when conducting human research. They insisted that the dangers of natural disease should outweigh the risks of a medical intervention, and they struggled to assess accurately the relative hazards. The book explores this logic of risk in immunization controversies extending as far back as the eighteenth century. Then, focusing on the period between 1930 and 1960, it shows how research physicians and their sponsors debated the moral quandaries involved in moving vaccine use from the laboratory to the clinic. This work describes the efforts of clinical investigators to balance the benefits and dangers of untested vaccines, to respond to popular sentiment about medical hazards, and to present strategically risk-laden research to sponsors and the public.