Drawing on a research collaboration between a group of medical physicists and social scientists, this paper aims to explore female volunteers’ experiences of participating in a project for developing a new breast disease diagnostic technology using an optical imaging system. In order to understand how these women make sense of being a volunteer, we examine the complexities of their experiences in this type of research setting through an empirically-based study involving participant observation and semi-structured interviews with the volunteers. Traditionally, volunteers are constructed as passive research material. In contrast, the women in our study are by no means docile bodies — but are active in deploying strategies that create opportunities to exert a level of control over perceived threats within the research encounter. We examine how volunteers translate these threats into ‘boundaries’ about what is and is not acceptable or permissible within this environment, paying particular attention to boundary setting around participation, and invasions of the body (such as pain, touch and exposure, and physical safety), and exploring the strategies volunteers draw on to counter perceived threats to their bodies.