CLAYTON, Missouri — Bayer is suing insurance giant AIG, alleging decades-old insurance policies should have helped cover billions of dollars in legal costs from Roundup and PCB cases now being shouldered by the German company.

In the suit, Bayer alleges that AIG and subsidiaries have refused “to honor their contractual obligations to provide insurance coverage for numerous lawsuits filed against Monsanto,” the Creve Coeur-based firm bought by Bayer in 2018.

"Thus far, more than 56,000 lawsuits have been filed against Monsanto by more than 149,000 plaintiffs," the suit says. "At present, approximately 50,000 of those lawsuits remain pending. Thousands of additional lawsuits may be filed against Monsanto in the future."

The new lawsuit focuses on insurance coverage that it says Monsanto purchased from AIG decades ago. In recent years, the weight of the Roundup litigation and other claims have crippled Bayer, cratering its stock price and fueling investors' calls for the company to split off its besieged agriculture arm that features Monsanto's legacy products.

The new suit, experts say, signals Bayer's willingness to pursue insurance companies to help cover a financial catastrophe that has reached roughly $12 billion in payouts and includes tens of thousands of cases.

“This is a fascinating case,” said Renee Henson, a University of Missouri law professor with expertise in product liability and insurance cases. “It’ll be interesting to see where it goes.... There is so much on the line, potentially.”

AIG did not respond to a request for comment. Bayer declined to provide additional information about the case.

When it purchased Monsanto, Bayer inherited a suite of liabilities, just in time for an avalanche of lawsuits centered around products Monsanto had sold, including Roundup and PCBs.

Roundup is a bestselling weedkiller that has sparked personal injury claims alleging that its active ingredient, glyphosate, caused health issues like cancer.

PCBs, or polychlorinated biphenyls, are chemicals with a variety of harmful health impacts that were used for decades in electrical and buildings products, before U.S. production was banned in 1979. Wide-ranging PCB contamination has triggered property damage lawsuits and cleanup efforts in settings that commonly include school buildings and public bodies of water.

Bayer's suit against AIG, filed last week in St. Louis County court, focuses on those two kinds of cases: the personal injury cases related to Roundup, and the property damage cases tied to PCB contamination.

The suit says that, from 1967 to 1986, Monsanto purchased millions of dollars in general liability coverage from AIG and other insurers, which the suit alleges are AIG subsidiaries, to protect against such claims.

The financial toll from both types of lawsuits reaches into the billions of dollars, the suit says.

Since 2020, Bayer has paid more than $10 billion just to settle cases about Roundup, for instance. And as of the end of last year, the company had paid more than $1.9 billion for PCB cases, the suit says.

The past insurance, it alleges, “are all excess liability policies, which were purchased as protection against, among other things, the risk of mass tort liability arising from the use of products, a significant risk for any major manufacturing company.”

The suit does not specify the total compensation sought, saying that the exact amount of damages would be determined at trial. But it argues that the insurers “are obligated to reimburse Monsanto” up to the insurance policies’ applicable limits, as well as for costs that the company “incurs to defend itself” against the underlying claims.

In July 2024, Bayer submitted a demand for payment to AIG related to Roundup costs, the suit says. It did the same for PCB costs in October 2024, and has since provided updated totals, according to the lawsuit.

The suit said the company has shouldered costs that “exceed the limits” of the relevant insurance policies.

Legal experts said the lawsuit is an unsurprising move from Bayer.

It’s not unusual for manufacturers to seek recovery from insurers in similar situations, said Henson, the Mizzou professor.

“The novelty here, of course, is the scale of the Roundup litigation,” she said.

A lot will hinge on factors like the timing of PCB contamination, when exposure to products like Roundup happened, and how certain insurance terms — like an “occurrence” — are defined, said Michael Green, a visiting professor at Washington University’s School of Law with areas of expertise in product liability and insurance cases.

“There’s a lot of case law on the question of when the occurrence occurred, from initial exposure, to the entire period you were exposed, to when the disease was determined,” he said.

Generally, Green said, courts tend to be sympathetic to insurance holders, and rule in ways that maximize coverage.

Experts cautioned that it’s far too soon to weigh the merits of the case.

Still, they predicted this case might not be the only one of its kind from Bayer, explaining that it’s possible — or even likely — that similar lawsuits could emerge against other insurers, too.

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