The present invention relates to a monoclonal antibody suitable for monitoring the activity of systems involving protein C inhibitor, a method for preparation of the monoclonal antibody, a method for monitoring the activity of systems involving protein C inhibitor and a method for diagnosis of e.g. venous thrombosis, wherein the monoclonal antibody is utilized. The monoclonal antibody is suitable for monitoring the activity of systems involving protein C inhibitor, and it has specific affinity for both i) a complex between a serine proteinase and an inhibitor thereof, and ii) a cleaved and uncomplexed form of the inhibitor, but has substantially no specific affinity for the inhibitor in its uncleaved and uncomplexed form; or a derivative thereof having the same biological activity.