In the UK the number of state social workers supplied by independent employment agencies has increased significantly since the mid-1990s. Although state sectors of welfare such as education and health have always relied upon a steady supply of locum staff, there is no such tradition within social work. This paper explores some of the ethical tensions that have become apparent with the expansion of contingency social work. Questions remain about the ‘opportunity cost’ of agency social work, as well as subsequent problems of recruitment, access to training, collegiate relations and any impact upon service users. The paper notes tensions that persist between a push for more flexible labour, the ethics of practitioners and the needs of service users. Recent policy responses which aim to push responsibility back to local authorities and social work managers are also questioned.