SAN FRANCISCO (CN) – Monsanto’s attorneys failed Thursday to undermine a New York toxicologist who says the company’s Roundup weed killer caused a groundskeeper’s deadly lymphoma, in a contentious California jury trial over the herbicide’s alleged carcinogenicity.

DeWayne Johnson, 46, sued Monsanto in 2016 after being diagnosed with a cutaneous form of non-Hodgkin lymphoma that caused cancerous lesions to form over most of his body. He says he developed symptoms after he was twice drenched in Roundup while spraying schoolyards for his job with the school district in Benicia, a suburb of San Francisco.

He also claims Monsanto has known for decades that Roundup is carcinogenic but didn’t disclose it for fear of disrupting its multi-billion dollar global business.

Throughout the trial, Monsanto’s defense strategy has centered around discrediting scientific experts testifying on Johnson’s behalf by suggesting they were paid to lie to the jury about Roundup’s carcinogenicity.

The strategy has seemingly worked, with Monsanto flagging six instances in which an expert testified differently on the stand than he had in his depositions, an instance in which a second expert omitted key details from his testimony showing Roundup couldn’t have caused Johnson’s cancer; and an instance in which a third expert purportedly lied publicly about his paid consulting work on glyphosate, the controversial active ingredient in Roundup.

But Winston & Strawn attorney George Lombardi, who represents Monsanto, was unable to decisively paint toxicologist William Sawyer in the same light when he noted that Sawyer answered a question differently Thursday than he had in a recent 14-hour deposition.

When asked whether glyphosate bioaccumulates in the body, Sawyer said it does not, but that it does bioaccumulate in the skin, and from there absorbed into the body.

In his deposition, however, Sawyer said glyphosate “does not bioaccumulate. No, at least not in the body.”

Lombardi suggested that Sawyer had lied on the stand. “You were asked the same question at your deposition under oath and you gave a different answer,” he said.

Nonetheless, it was unclear whether Sawyer had in fact been untruthful, or whether he meant that glyphosate bioaccumulates in the skin but not inside the body.

Sawyer also survived Lombardi’s attempt to discredit his opinions by referencing a study that found that the skin absorbs glyphosate at high rates, the results of which he said boost Johnson’s contention that his dermal exposure to Roundup caused his skin-based lymphoma.

Judge Suzanne Ramos Bolanos had barred Sawyer from referencing the study, however, and Lombardi asked her for a curative jury instruction after trial adjourned for the day.

“Nothing was an accident here,” he said, noting that Sawyer has testified as an expert witness in 20 trials. “It was blatant, it was repeated, and it went right up until the last answer to the last question.”

Sawyer, a toxicology consultant with New York-based firm Toxicology Consultants & Assessment Specialists LLC, has testified for both plaintiffs and defendants, and has consulted for U.S. Attorney offices and the U.S. Navy, according to his company’s website.

Brent Wisner, Johnson’s attorney with Baum Hedlund Aristei Goldman, countered that the testimony was “100 percent your doing.”

Lombardi, he said, had suggested that Sawyer wasn’t giving the jury the “full story” about skin-absorption rates, so Sawyer referenced the prohibited study to avoid looking “incapable.”

“Mr. Lombardi had every opportunity not to go there,” Wisner said, adding that Lombardi was merely “upset his cross [-examination] was not very effective on this witness.”

Bolanos instructed the parties to discuss a potential curative jury instruction Thursday evening.

Earlier in the day, Sawyer also testified that Roundup caused Johnson’s illness because the safety instructions included with the herbicide don’t tell users how to protect themselves while spraying it.

Monsanto, he said, determined internally that users must wear waterproof clothing while spraying, yet didn’t include any instructions with the product about protective clothing.

Johnson wore a suit only designed to keep out dust, unaware that he needed a waterproof suit, according to his testimony Monday.

Sawyer said because Roundup irritates the skin, those regularly exposed to it like Johnson develop greater skin permeability, increasing the amount of Roundup absorbed into the body.

Johnson also sprayed an “extraordinarily heavy” amount of 50 gallons of Roundup per hour during two summer spraying seasons, he said, making him particularly susceptible to developing lymphoma because Roundup is more carcinogenic than pure glyphosate due to added surfactants that help the chemical spread across plant surfaces.

Johnson’s lawyers argue that Roundup is more carcinogenic than glyphosate alone because the surfactants boost the chemical’s cancer-causing properties.

In 2016, the European Union banned a suspect surfactant called POE-tallowamine in glyphosate-based herbicides, citing concerns over toxicity. It is still used in Roundup products sold in the United States.

Sawyer told the jury that Monsanto has never done carcinogenicity tests on POE-tallowamine. But when Lombardi countered that the Environmental Protection Agency requires such testing, Sawyer acknowledged the agency had determined POE-tallowamine doesn’t pose a risk to human health.

But he cautioned the EPA didn’t run long-term animal studies and based its finding on computer simulations, which he slammed as a “crude screening test” that “doesn’t tell us anything for sure.”

“It’s a very conditional conclusion of the EPA,” he said. “I would not frame that in terms of some kind of equivocal answer by EPA.”

Bolanos barred testimony about the European Union ban on POE-tallowamine, ruling that it was irrelevant to the case and would confuse the jury.

Glyphosate is the most widely used agrichemical in history. Monsanto introduced it in 1974, and its use exploded in 1996 after the company began selling “Roundup-ready” seeds engineered to resist the herbicide. More than 2.6 billion pounds of the chemical were spread on U.S. farmlands and yards between 1992 and 2012, according to the U.S. Geological Survey.

Testimony continues Friday.