Macro-analyses on the regulation of new biomedical objects tend to focus on discursive structures and legislative categories in science policy debates at national and cross-national levels, but overlook how actors engage in regulatory practices on an everyday basis. Based on data from ethnographic fieldwork in British and German clinics, and 32 interviews with medical staff, this article provides an insight into the regulation of adult stem cell research and its clinical implementation. The argument illustrates the enactment of regulation at different stages and highlights the accompanying interpretative strategies employed by the medical personnel involved in the management of clinical trials using patients' own (autologous) stem cells to regenerate damaged cardiac tissue. We argue that the implementation of regulation is a practical accomplishment in both national contexts. The complexities present in this process are instanced by the gradual crystallisation of practices within the organisation of clinical trials. This crystallisation is dependent on exchanges between members of medical teams and external agencies, and is set within a strategic ordering of regulatory measures that are mobilised to legitimise clinical research and reinforce professional interests.