Based on the anahsis ofthe Medical and Public Health Research programme (Commis- sion of the European Communities), the paper shows how new scientific communities are created in response to the HIV/AIDS problem. W e anahze how actors are mobilized (three mobilization modes: public impetus, scientists ‘initiative, scientists’ initiative with public networking) and how their work is coordinated. We defend the hypothesis that these new scientific communities are flexible cooperation networks. In the case of AIDS research, there are onb a limited number o f network types (the data collection structure, the forum, the thematic partition with harmonization o f research practices, the starred around a central facilip). The coordination of these scientific cooperative networks passes through fixed and circulating intermediaries. The management of these intermediaries is a major activity for involved actors. Thus, our aim is not to study the wider social context, but to analyze networking in response to policy, initiatives.