Controversies about global clinical trials, particularly HIV trials, tend to be framed in terms of ethics. In this article, I explore debates about ethics in the Cambodia Pre-Exposure Prophylaxis trial, which was designed to test the safety and efficacy of tenofovir as a prevention for HIV infection. Bringing together studies of public participation in science with studies of bioethics, I show how activists around the Cambodian Pre-Exposure Prophylaxis trial circulated and provoked debates about standards of research ethics, as opposed to research methodology. This postcolonial bioethics was configured through the circulation of and debate about ethics guidelines, and historically and culturally specific relations of vulnerability and responsibility between foreigners and Cambodians and between Cambodian leaders and Cambodian subjects. I argue that this shift in the object of ethical concern, from the experimental human subject to the relation between subjects and researchers, illustrates how a postcolonial field of articulation reformulates classical bioethics.