We consider antitrust enforcement within the adversarial model used by the United States. We show that, under the adversarial system, the Antitrust Authority may try to prohibit mergers also in those cases in which litigation is ine? cient. Even if market concentration and technological disadvantages lead to a significant welfare reduction after merger, from society´s perspective the agency´s lawsuit may be ine? cient. We can show that these ine? ciencies may be aggravated if the takeover is hostile.