This paper discusses the discourse of ‘participation’ in the context of genetic databases. Focusing on UK Biobank, it suggests that this discourse can be seen as a reflexive institutional response to public ambivalence towards science and expertise. Drawing on empirical evidence from focus groups, I explore how people from various backgrounds constructed and contested two different kinds of participation in UK Biobank. The first relates to people providing research materials to genetic databases and the second to people becoming ‘co-decision makers’ in these projects. My analysis highlights how focus groups positioned themselves and others in relation to UK Biobank and drew on a variety of ‘discursive repertoires’, such as altruism, public ignorance, expertise, and lay empowerment. I conclude that discourses of participation reflect the way people position themselves and others - as experts, publics, patients, or research subjects - in relation to the opportunities and uncertainties of new biomedical research.