The authors study how an aerosol technology research group constructed a research agenda for itself and how its activity was changed in the process. The group's research agenda was heterogeneous, comprising several research areas in which the knowledge of aerosols was applied in different industrial contexts. The authors analyze the development of one of these areas, the research on the production of ultrafine particles from 1992 to 1997, employing the concept of mediated activity that has been developed in the cultural-historical (...) theory of activity. In the analysis, the activity is seen as the coevolution of the subject, the means, and the research object and the networks of collaboration with other activities. The applications developed in the collaborative networks significantly influence the identity and learning of the researchers, the construction of tools and instruments, and the selection of the phenomena to be studied. (shrink)