The main research question in this article is how merger integrations are influenced by local industrial relations. These are explored and illuminated by combining perspectives on industrial relations and perspectives on mergers and acquisitions. The M&A field of research has been dominated by organization and management perspectives, and industrial relations are seldom brought into the studies. Studies in the field of industrial relations are mostly concerned with institutional levels. Qualitative data and analyses of four integration processes together with 108 personal interviews are presented. By focusing on industrial relations at local company levels in the integration of merging partners new insights are sought for both fields of research. Local implementations of industrial relations contribute to the explanation of integration outcomes. Merger integration implies the integration of different traditions of union and employee–management collaboration that in essence may merge the collaboration cultures into something new.