Contract enforcement is acknowledged as a major issue in Law and in Economics. Contrasting substitution and complementary perspectives with respect to the role of private versus public enforcement institutions, this article analyses how contract law can support private institutions, and enhance economic efficiency. With multilateral agreements at stake, self-regulation and reputation mechanisms at the core of private ordering have limitations that collective organizations backed by the Law help to overcome. The analysis is substantiated by empirical data from the cattle industry. Our results suggest the need for a broader approach to contract regulation by legal scholars and antitrust-authorities.