In the context of the high-profile controversy that has unfolded in the UK around the measles, mumps and rubella (MMR) vaccine and its possible adverse effects, this paper explores how parents in Brighton, southern England, are thinking about MMR for their own children. Research focusing on parents' engagement with MMR has been dominated by analysis of the proximate influences on their choices, and in particular scientific and media information, which have led health policy to focus on information and education campaigns. This paper reports ethnographic work including narratives by mothers in Brighton. Our work questions such reasoning in showing how wider personal and social issues shape parents' immunisation actions. The narratives by mothers show how practices around MMR are shaped by personal histories, by birth experiences and related feelings of control, by family health histories, by their readings of their child's health and particular strengths and vulnerabilities, by particular engagements with health services, by processes building or undermining confidence, and by friendships and conversations with others, which are themselves shaped by wider social differences and transformations. Although many see vaccination as a personal decision which must respond to the particularities of a child's immune system, 'MMR talk', which affirms these conceptualisations, has become a social phenomenon in itself. These perspectives suggest ways in which people's engagements with MMR reflect wider changes in their relations with science and the state.