Over the past decade and a half, an increasing number of clinical trials have been conducted in Cambodia, making the country a source of data about HIV, malaria, and other conditions. Has Cambodia also shaped the practice of clinical trials? If so, how, and to what effect? I take up this question of how contexts shape scientific practices by exploring Cambodia’s first experimental trial, the Cambodia Pre-Exposure Prophylaxis trial. The trial was designed to test the safety and efficacy of tenofovir as a prevention for HIV infection, and was cancelled in 2004. Debates about ethics of the trial invoked international bioethics guidelines, as well as historical relations of vulnerability and responsibility between foreigners and Cambodians, and between Cambodian leaders and Cambodian subjects. These debates shifted the object of concern in classical bioethics, from the experimental human subject to the relation between subjects and researchers. This case illuminates how a postcolonial field of articulation in Cambodia reformulates classical bioethics.