This paper examines how the presence of an antitrust authority (AA) affects market-sharing agreements made by firms. These agreements prevent firms from entering each other’s markets. The set of agreements defines a collusive network, which is pursued by antitrust authorities. This paper shows that in the absence of an AA, a network is stable if its alliances are large enough, and in the presence of an AA, more competitive structures can be sustained through bilateral agreements. Antitrust laws may have a procompetitive effect, as they give firms in large alliances more incentives to cut their agreements at once.