Kennelly ruled that, if what the providers have said is true, the health plans may have used MultiPlan's out-of-network claim repricing service to find out what competitors were paying and to align their own out-of-network reimbursement rates with competitors' rates.

Helping health plans align their reimbursement rates that way could be a form of price-fixing that would violate federal and state antitrust laws and state consumer protection laws, Kennelly wrote in an opinion explaining his ruling.

Kennelly said that his ruling does not necessarily mean that the facts in the plaintiffs' allegations are correct.

"At the motion to dismiss stage, the court accepts all well-pled factual allegations as true," Kennelly wrote in the opinion.

The next steps: Kennelly asked the parties to agree on a schedule for "discovery" and other pretrial proceedings by June 10.

The discovery process gives the parties in a lawsuit a chance to get records and other information from each other.

The parties are supposed to hold a video case management conference June 17.

The backdrop: Kennelly, a judge in the U.S. District Court for the Northern District of Illinois, is presiding over In re MultiPlan Health Insurance Provider Litigation.

In re MultiPlan is a "multidistrict litigation" proceeding, or MDL.

The courts established the MDL to handle a torrent of lawsuits filed by providers who were angry about MultiPlan's repricing strategy.

Related: Panel consolidates Health Care Providers v. Payers in Illinois

The providers have accused MultiPlan and the health plans, including many health coverage providers that were acting as administrators for employers' self-insured health plans, of conspiring to underpay providers by about $19 billion per year.

MultiPlan and the health plans have argued that they are using public data to fight skyrocketing health care costs.

Officials in the U.S. Justice Department said in March that they would support the providers.

Opinion details: Kennelly noted that the defendants say the plaintiffs have not asserted a viable federal antitrust claim partly because they have not plausibly alleged that they have suffered a direct injury.

"The defendants argue that providers are not directly injured by the alleged third-party payor agreement to fix prices because they can always seek full payment from the patient," Kennelly wrote.

In this case, however, the providers have agreed to accept MultiPlan's repricing and not bill the patients for any unpaid portions of the bills, Kennelly wrote.

The defendants have also argued that the prices the providers have been getting have been reasonable, but "price-fixing agreements between competitors cannot be justified at all, let alone by the argument that currently-fixed prices are more 'reasonable,'" Kennelly said.

Even if a price-fixing agreement seems to lower consumer prices, it "cannot be justified, period," Kennelly added.