In the latest example of an outside force making a move into healthcare, retail giant Walmart—the largest company in the world by revenue—is reportedly in preliminary discussions to acquire health insurer Humana, according to the Wall Street Journal.

WSJ reported there’s no guarantee the two companies will reach a deal and a variety of options are being explored, including an acquisition. Buying Humana wouldn’t come cheap—it's currently valued at $37 billion.

Walmart has already made some expansions into care delivery with its existing primary care clinics and has announced plans to bring lab testing services into some stores. It also has pharmacies in most of its 4,700 U.S. stores. Adding the second-largest Medicare Advantage insurer in Humana, WSJ reported, would give Walmart an edge with seniors.

If the deal was completed, it would dwarf the other megadeals announced in recent months. Walmart and Humana would have a combined annual revenue of $554 billion—more than double the $246 billion in revenue from both CVS and Aetna and well above the $142 billion from a merged Cigna and Express Scripts.

These vertical integrations in healthcare haven’t been challenged by the U.S. Department of Justice (DOJ), which successfully blocked some of these same insurers from merging last year. That may change, however, depending on the DOJ’s ongoing case against AT&T and Time Warner.

The American Antitrust Institute has made an argument for the DOJ to take on those proposed mergers, which would have ramifications for Walmart if it moves to buy Humana. In a letter to the DOJ’s antitrust division, the institute said these combinations could “trigger a fundamental restructuring of the U.S. healthcare system” and create a much stronger possibility of “anticompetitive coordination.”

“While vertical mergers do not eliminate rivals and increase market concentration, they can enhance the ability and/or incentive for a merged firm to behave in ways that harms competition at a horizontal level,” the AAI wrote. “By combining inputs with distribution, for example, a vertical merger can enhance incentives for the merged firm to exclude its downstream or upstream rivals, either by raising their costs or cutting off their access to critical resources.”