US jury finds Roundup caused man's cancer

A US jury has found that Roundup, one of the world's most widely-used weed killers, was a "substantial factor" in the cancer of California man Edwin Hardeman.

A jury has found one of the most widely-used weed killers was responsible for a US man's cancer. (AAP)

A US jury has found Bayer AG's glyphosate-based weed killer caused a man's non-Hodgkin's lymphoma in a bellwether trial that may help determine the course of hundreds of similar cases.

The finding by the unanimous jury in San Francisco federal court on Tuesday clears the way for that same jury to determine if Bayer unit Monsanto is liable and must pay damages to California resident Edwin Hardeman in a second trial phase.

Bayer in a statement said it was disappointed with the jury's initial decision.

"We are confident the evidence in phase two will show that Monsanto's conduct has been appropriate and the company should not be liable for Mr Hardeman's cancer," the company said.

The case was only the second of some 11,200 Roundup lawsuits to go to trial in the US. Another California man was awarded $US289 million ($A407 million) in August after a state court jury found Roundup caused his cancer, sending Bayer shares plunging at the time. That award was later reduced to $US78 million and is on appeal.

Hardeman's case is expected to help determine the range of damages and define settlement options for more than 760 Roundup cases consolidated in the federal court in San Francisco.

Bayer denies allegations that Roundup, or glyphosate, cause cancer. It says decades of studies and regulatory evaluations, primarily of real-world human exposure data, have shown the weed killer to be safe for human use.

After five days of deliberations over scientific evidence presented during the trial, the jury on Tuesday found Roundup, one of the world's most widely-used weed killers, to be a "substantial factor" in causing Hardeman's cancer.

During the first phase, Hardeman's lawyers were not allowed to present evidence allegedly showing the company's efforts to influence scientists, regulators and the public about the safety of its products. They will be able to present that evidence in the second phase.

Hardeman's lawyers, Aimee Wagstaff and Jennifer Moore, said they look forward to presenting that evidence to the jury to hold Monsanto accountable.

"Now we can focus on the evidence that Monsanto has not taken a responsible, objective approach to the safety of Roundup," they said in a statement.