This paper considers how parents engage with a large, internationally supported childhood pneumococcal vaccine trial in The Gambia. Current analysis and professional reflection on public engagement is strongly shaped by the imperatives of public health and research institutions, and is thus couched in terms of acceptance and refusal, and 'informed consent'. In contrast Gambian parents' perspectives on the trial are couched in conceptual and experiential terms that are linked to their wider dilemmas of raising infants amidst the hazards of globally connected village life. Ethnographic research reveals how for most parents, longer-term experiences of the organization managing the research (the UK Medical Research Council Laboratories in The Gambia) as a health-providing institution override their reflection on trial-specific aims. A decision to participate in the trial involves a perceived balance of benefit and danger -- in the extreme, of free medical treatment, versus one's child being drained of blood for sale to Europe. Social relations (especially gender relations) shape this calculus and study participation. This case suggests how the idea of 'public engagement with science' in a globalized context might be recast, with implications for debates in biomedical ethics, and the sustainability of public participation in medical research.