Antitrust laws impact mergers and acquisitions by requiring most large deals to be reported to federal regulators before they close, forbidding any acquisition whose effect “may be substantially to lessen competition, or to tend to create a monopoly,” and giving the government the power to demand changes, force divestitures, or sue to block the transaction outright.1Office of the Law Revision Counsel. 15 U.S. Code 18 – Acquisition by One Corporation of Stock of Another The word “may” is doing serious work in that statute. Regulators do not have to prove a deal will harm competition. A reasonable probability is enough.
Two agencies share the job. The Department of Justice Antitrust Division and the Federal Trade Commission split incoming reviews based on which one has deeper industry expertise, and whichever agency takes the file runs the whole investigation.2Federal Trade Commission. Premerger Notification and the Merger Review Process
When Antitrust Law Applies to a Deal
The Hart-Scott-Rodino Antitrust Improvements Act created the premerger notification system. Deals that meet the financial tests must be filed with both agencies, and the parties have to wait out a statutory review period before they can close.3Office of the Law Revision Counsel. 15 U.S. Code 18a – Premerger Notification and Waiting Period
For 2026, two tests run together. The size-of-transaction test asks whether the value of the acquired voting securities or assets exceeds $133.9 million. If it does, the size-of-person test looks at the parties: generally one side must have at least $267.8 million in total assets or annual net sales, and the other at least $26.8 million. Above $535.5 million, the size-of-person test drops out and filing is mandatory regardless of the parties’ size.4Federal Trade Commission. Current Thresholds
The Act does exempt certain foreign-asset acquisitions and acquisitions in the ordinary course of business, among others, but the exemptions are narrow and the safer assumption is that a qualifying deal must be filed. Failing to file is not treated as a paperwork slip. The statute imposes a civil penalty currently around $51,744 per day of violation, and officers, directors, and partners can be personally liable.3Office of the Law Revision Counsel. 15 U.S. Code 18a – Premerger Notification and Waiting Period
What the Review Process Looks Like
Once the Premerger Notification and Report Form is submitted, the statutory waiting period starts. Most deals move through quickly.
Phase I
The standard waiting period is 30 calendar days from receipt of the completed filings, cut to 15 for cash tender offers and bankruptcy acquisitions.5Federal Trade Commission. Merger Review Agency staff review the filing, market data, and internal documents for red flags. The overwhelming majority of transactions end here — either the clock runs out or the agency grants early termination. Early termination was suspended for a period and has since been reinstated as a discretionary practice.6Federal Trade Commission. Granting of Requests for Early Termination of the Waiting Period Under the Premerger Notification Rules
Phase II and the Second Request
If the initial review turns up potential competitive problems, the agency issues a Second Request, a formal demand for additional information. That automatically extends the waiting period. Second Requests typically call for large volumes of internal documents, sales and customer data, and economic analyses, and complying often takes many months.
After both parties certify substantial compliance, the agency gets another 30 days on most deals, or 10 days for cash tender offers and bankruptcy sales (with the clock starting when the buyer complies).2Federal Trade Commission. Premerger Notification and the Merger Review Process In practice, the compliance effort is what drags the calendar, not the statutory clock.
How Regulators Measure Competitive Harm
The substantive question underneath every antitrust review is whether the deal will meaningfully reduce competition. Answering it takes three steps.
Defining the Relevant Market
Regulators first draw a boundary. The product market covers every product or service customers would realistically switch to if the merged firm raised prices. The geographic market captures where those customers can practically shop. A rural hospital merger sits in a much smaller geographic market than a merger between cloud computing providers, and that framing shapes everything that follows.
Concentration and the Structural Presumption
With the market defined, regulators score concentration using the Herfindahl-Hirschman Index. Squaring each competitor’s share and adding the results produces a number: 1,000 for a market split evenly among ten firms, 10,000 for a pure monopoly.7U.S. Department of Justice. Herfindahl-Hirschman Index
Under the 2023 Merger Guidelines, a post-merger HHI above 1,800 counts as highly concentrated. If the deal also raises the HHI by more than 100 points, the agencies presume the merger is likely to substantially lessen competition. A second trigger applies independently: a combined share above 30 percent plus an HHI increase over 100 points also creates a structural presumption.8Federal Trade Commission. 2023 Merger Guidelines The presumption is not the end of the analysis. It shifts the burden to the parties to show the numbers overstate the concern.
How the Theory of Harm Changes with Deal Type
The relationship between the merging companies shapes what regulators worry about.
In a horizontal merger, two direct competitors combine. The clearest risk is that the merged firm can raise prices on its own because customers have lost an alternative. Even when unilateral pricing power is limited, fewer competitors can make it easier for the survivors to coordinate tacitly on price or output.
Vertical mergers involve companies at different levels of the same supply chain, like a manufacturer buying a key parts supplier. The core concern is foreclosure: the merged entity denying or degrading rivals’ access to a critical input or distribution channel. The 2023 Merger Guidelines weigh how important the input is, whether substitutes exist, and whether the merged firm has both ability and incentive to restrict access.8Federal Trade Commission. 2023 Merger Guidelines When the merged firm would control more than 50 percent of the related product market, the agencies generally infer foreclosure power.
Conglomerate mergers pair firms in related but non-competing markets. The concern is that dominance in one area can be leveraged into another through bundling, tying, or preferential access to shared customers. These deals get less scrutiny but are not exempt.
Labor markets now sit on the same map. The 2023 Guidelines treat them as buyer markets and apply the same framework, so a deal between two firms competing for the same workers can be challenged on the theory that it will suppress wages, benefits, or working conditions.9United States Department of Justice. Guideline 10 – When a Merger Involves Competing Buyers Industries where workers face high switching costs — specialized skills, geographic constraints, licensing — draw particular attention.
What Merging Parties Can Argue Back
A structural presumption is not the end of the deal. Two defenses come up regularly.
The first is procompetitive efficiencies. The parties argue that the merger will produce cost savings, better products, or innovations that neither firm could achieve alone, large enough to offset the competitive harm. To count, efficiencies must be merger-specific, verifiable with real data, and likely to benefit consumers rather than only the merged firm’s shareholders. Regulators are skeptical of these claims, and generic promises of synergy carry no weight.
The second is the failing firm defense. If the target is truly on the brink of collapse, the parties can argue the deal should proceed because the alternative is losing the firm entirely. The Supreme Court has set a high bar, requiring three showings: a grave probability of business failure, no viable reorganization through bankruptcy, and no other willing purchaser after a good-faith search.10United States Department of Justice. Rebuttal Evidence Showing That No Substantial Lessening of Competition Declining sales or net losses are not enough; the firm must be unable to meet near-term financial obligations. Any alternative offer above the liquidation value of the assets qualifies as a reasonable alternative, which typically ends the defense.
How the Review Ends
An antitrust review closes in one of three ways.
Clearance. Most deals clear without conditions, either by expiration of the waiting period or by early termination, and the parties close.
Negotiated settlement. When the agency identifies competitive harm but believes it can be fixed, the parties negotiate a remedy. The FTC uses consent orders; the DOJ uses consent decrees entered as court judgments. Either way, the merged company takes on binding obligations as the price of closing.
The most common remedy is divestiture — selling specific business units, assets, or product lines to an independent buyer who can preserve competition. The FTC requires the buyer to be both financially viable and competitively capable, and can reject one that isn’t.11Federal Trade Commission. Negotiating Merger Remedies When what’s being divested is less than a full standalone business, the agency typically insists the buyer be identified and approved before closing.
Behavioral remedies appear less often, alone or alongside structural ones. They require the merged firm to change future conduct: licensing technology to competitors, keeping distribution channels open, or firewalling competitively sensitive information between business units. Agencies generally prefer structural fixes because a divestiture is permanent and self-executing, while behavioral commitments demand ongoing monitoring, often through a monitor trustee paid by the merged company but reporting to the agency.
Litigation. If no acceptable settlement emerges, the agency sues. The FTC seeks a preliminary injunction under Section 13(b) of the FTC Act; the DOJ files directly under the Clayton Act. In either court, the agency must show the merger is likely to substantially lessen competition.5Federal Trade Commission. Merger Review As a practical matter, a preliminary injunction usually kills the deal. Even parties confident they would win at trial rarely survive the delay and financing uncertainty a lost injunction creates.
What Parties Cannot Do Before Closing
Filing the HSR notification does not permit the companies to start operating as one. Until the waiting period ends and the deal formally closes, they must continue running as separate, independent competitors. Transferring operational control early, sharing competitively sensitive pricing or strategy data, or effectively integrating before clearance is called gun-jumping, and it violates the HSR Act. If the coordination looks like an agreement between competitors to fix prices or divide markets, it can also trigger Sherman Act liability.
The exposure is significant. HSR violations carry the per-day civil penalty noted above, and the FTC has imposed multimillion-dollar fines for gun-jumping. If conduct crosses into Sherman Act territory, criminal fines can reach $100 million per company, and private plaintiffs can pursue treble damages.
State Attorneys General Can Act Independently
Federal clearance is not the only variable. State attorneys general have independent authority under federal and state antitrust statutes to investigate and challenge mergers, and a deal that clears the DOJ or FTC can still face a separate state suit. Challenges are more common in industries where competitive effects are felt locally, healthcare and energy in particular, and state action has become a more frequent part of deal timeline and risk planning in recent years.