This paper considers how internationally supported medical research is understood and interpreted by its actual and potential study subjects, exposing the limits to bioethical discourses amidst economic inequalities and contrasting socio-cultural worlds. It focuses on the Medical Research Council (MRC) Laboratories in The Gambia and particularly their Pneumococcal Vaccine Trial (PVT) that was conducted jointly with the Gambian government during 2001-2004. In many respects this was an exemplar of international best practice in trial communication and informed consent procedures. Yet ethnographic and survey research finds that Gambian parents' perspectives on participation are shaped not by trial specificities, but by broader, historically shaped views and experiences of the MRC as an institution. There is a pervasive view that the MRC offers good, free medication to participants, but that it also 'steals blood'. Widespread concerns with blood-stealing emerge from local frames of understanding in which blood is treated as a tradeable good, in which blood accumulation and depletion in bodily processes relates to its exchange in hospital and medical research practices, and in which transactions can be more or less (un)reasonable. Yet such thinking, rooted in an 'economy of blood', has been overlooked by medical research staff and indeed by historians and anthropologists of Africa whose analyses of blood-stealing have been overly transfixed on rumour and the occult. This paper argues that such cultural framings, which guide local critical commentary on trans-national research orders, require serious attention and need to inform open dialogues between scientists and the public if medical research in resource-poor settings is to continue to be sustainable and politically legitimate.