A necessity for case studies describing daily operations in high-risks industries has been highlighted. Despite the central role of OHS practitioners managing safety constraints, little research on their activities has been car-ried out. This paper presents a long term in-depth case study conducted in a pharmaceutical chemistry plant. The objective of the study is to provide a description of the activities, roles and influence of OSH practitioners in or-der to identify how they contribute to the construct of safety within their organization. In order to address these issues the theoretical and methodological frameworks of ergonomics and sociology of organizations are used.