California's “Mini-HSR” Antitrust Filing Arrives in 2027: A Practical Compliance Checklist
Authors: Paul Moore and Steven Cernak
If your company files Hart-Scott-Rodino paperwork on a deal with any meaningful California footprint, you will soon have a second filing obligation — to California Attorney General Rob Bonta's office. SB 25, the California Uniform Antitrust Premerger Notification Act, was signed in February 2026 and applies to any HSR notification filed on or after January 1, 2027. It is not a heavy lift, but it is a mandatory one, and the deadlines are unforgiving.
California is the third state, following Washington and Colorado, to adopt a broad premerger notification statute modeled on the Uniform Law Commission's Uniform Antitrust Premerger Notification Act. (Indiana’s legislature has passed similar legislation but it is unclear if the bill has been signed or, if so, when it will come into effect.) Multistate filers will see a familiar pattern. But California is by far the largest market to join, and that changes the calculus for almost every sizable transaction.
Who gets caught. SB 25 reaches a party to an HSR-reportable deal if either of two things is true: the party has its principal place of business in California, or the party (together with entities it controls) had California net sales of the goods or services involved in the transaction equal to at least 20% of the federal size-of-transaction threshold. At today's thresholds that second prong kicks in at roughly $26.78 million in in-state sales — but that number floats with the annual February HSR adjustment, so confirm the operative figure each year rather than relying on a stale memo.
A trap worth flagging: each party tests the thresholds on its own. Unlike the federal filing, it is entirely possible that only one side of a deal owes a California filing. Don't assume your counterparty's analysis answers are yours.
What you actually have to do. Provide a copy of your federal HSR filing to the California AG within one business day of submitting it federally. That is a tight window, and it is the part that could catch deal teams off guard. If you qualify under the principal-place-of-business prong, you must also include the documentary attachments submitted with the federal form (e.g., the Item 4(c) and 4(d) materials). If you qualify only under the sales prong, you provide those materials only if the AG asks.
But how, exactly, you provide a copy of that HSR filing is yet to be determined. Washington and Colorado started with a combination of email inboxes and secure FTP links. In June 2026, Washington established a portal which requires a filer to provide information and then upload the required materials. California has not yet announced the mechanics of its system.
It's mandatory, but it won't hold up your deal. SB 25 is non-suspensory: there is no California waiting period, and you can close before the AG finishes any review. The filing is an information-sharing and early-coordination tool, not a state approval gate. That said, even though it is “non-suspensory” that does not mean the filing is optional. The AG is authorized to charge filing fees — which the office has signaled will be $1,000 for principal-place-of-business filers and $500 for sales-prong filers — and to impose fines of up to $25,000 per day for noncompliance. Missing the one-day deadline could expose a party to significant daily penalties.
Confidentiality, with an asterisk. Submissions are statutorily protected from public disclosure. But the statute lets the California AG share filings with other state attorneys general whose states have substantially similar laws. With Washington and Colorado already in, and New York, Hawaii, Indiana, West Virginia, and D.C. all weighing their own versions, plan on the realistic possibility that your filing reaches a coordinated group of state enforcers — not just one office.
In practice, parties are typically asked to grant waivers so that federal and state enforcers can share information and coordinate their review of a transaction. Because SB 25 compels the parties to hand the state a copy of their own filing, it does not itself implicate the federal confidentiality rules that limit what the FTC and DOJ may disclose. The open questions are how aggressively California will use its authority to share filings with other states, and whether parties will still be asked for waivers to enable substantive joint federal-state investigation.
Why this matters. Federal merger review is primarily conducted by the Department of Justice and the Federal Trade Commission, which focus on competition effects across national and regional markets and have the benefit of extensive merger-review experience, economists, and established investigative processes. State attorneys general often approach transactions differently, and while California has become a more active merger enforcer with a more extensive merger review process, it operates no federal-style clearance process today. States may place greater emphasis on local competitive effects, labor markets, healthcare access, consumer protection concerns, and the impact of a transaction on residents within their borders. State enforcers also are not required to defer to federal conclusions. A transaction that receives no federal challenge can still face scrutiny from one or more states, and state attorneys general increasingly coordinate investigations and litigation efforts. For filing parties, the lesson is that expiration of the federal waiting period does not necessarily end antitrust risk.
A short checklist for deal teams:
- Add a California-nexus screen to your standard HSR analysis, run separately for each filing party.
- Track the current 20%-of-threshold sales figure each year; don't hardcode $26.78 million.
- Review transaction-agreement filing covenants to ensure the California filing is incorporated into standard HSR workflows.
- Build the one-business-day California submission into your filing timeline now, not at signing.
- Pre-assemble the federal documentary materials so a principal-place-of-business filer can transmit them the same day.
- Budget for the modest fees, and treat the deadline as hard given the per-day penalty exposure.
Bona Law advises clients on federal and state merger notification requirements, including California's new SB 25 regime. To assess how these obligations apply to a pending or contemplated transaction, contact Paul Moore or Steve Cernak.
What to watch for: Filing an HSR in December 2026 escapes the California submission requirement whereas the same transaction filed in January 2027 must comply with this new regime. A withdraw-and-refile likely re-triggers the California filing.
The filing mechanics are the near-term task; the larger trajectory is what to watch. California's Law Revision Commission is separately weighing amendments to the Cartwright Act that would expand or clarify the state’s ability to challenge mergers substantively — and recent amendments already extended the Act to algorithmic pricing and lowered pleading standards. SB 25 is the modest, procedural first move. The substantive expansion is what to watch next. The near-term to-do list is short: (1) confirm scope for each party, (2) and build the one-day filing into your timeline before signing.