Contemporary systems of welfare are increasingly underpinned by the assumption that families should care for their dependent adult relatives. Yet, the burgeoning empirical literature in this area has given little attention to the ways in which family caregivers fit into the service system. Drawing on interactionist theories of the division of labour, this paper employs ethnographic data, taken from a recent study, to explore the ways in which formal and informal carers negotiate ‘care’ on an adult medical ward. It is argued that established family carers (referred to as expert carers) constitute a special case in understanding the negotiation of care in the hospital context because of the challenge they pose to fundamental features of the social organisation of the work: nurses’ control over caring processes, their claim to expertise and their license to define standards of care.