In this paper, we distinguish decisions about cooperation with medical research from decisions about research participation. We offer an empirical and theoretical exploration of why people in three different UK-based medical research projects chose to cooperate. Data analysis of the accounts of 128 participants across the three studies was based on the constant comparative method. Participants' cooperation was engaged by a perception that they would be contributing to the 'public good', but they also wanted to justify their decision as sensible and safe. Critical to their cooperation was their belief that researchers would fulfil their side of the cooperative bargain, by not exposing participants to risks of harm or exploitation. Although participants were generally unaware of the details of the regulatory regime for research, they demonstrated a generalised reliance on regulation as a feature of everyday life that would provide a safe context for cooperation. In their assessment of particular projects, participants made judgements about whether to cooperate based on more specific cues, which acted as signs to assure them that researchers shared their cooperative intentions. These cues included organisational and professional credentials, the role identities and perceived trustworthiness of those involved in recruiting to research, and visible signs of reasonable practice mandated by regulatory systems. Thus participants drew on their understandings of an institutional field that was much broader than that of research alone. We propose that the social organisation of research is fundamental to the judgements people make about cooperation with research. Cooperation may be a more useful way of thinking about how people come to engage in collaboratively oriented actions such as research participation, rather than currently dominant individualistic models. Attention to the institutional context of research is critical to understanding what makes cooperation possible, and has important implications for the design of regulatory regimes for research.