A pharmacology professor testifying in the U.S. government’s $280 billion suit against the cigarette industry said on Monday he quit working as an expert witness for a tobacco trade group after it pressured him to be more of an advocate.

Georgetown University professor Sorell Schwartz, who studies the effects of drugs, said the Tobacco Institute urged him and other researchers during the 1980s to “take a more advocative position” in addressing concerns about the hazards of secondhand smoke.

That position deviated from the original job “to have an open discussion of the science” that Schwartz and other academics at his consulting firm were hired to do, he said, adding that he stopped testifying on the institute’s behalf after three years in 1987.

In pre-written testimony before U.S. District Judge Gladys Kessler, Schwartz said that his group, the Center for Environmental Health and Human Toxicology, was supposed to review scientific data, including data on the effects of second-hand smoke on non-smokers.

Schwartz said he continued working as a consultant for years for other tobacco interests after he quit serving as the institute’s witness, including training and briefing others “who might be more amenable” to testifying.

Philip Morris, a subsidiary of Altria Group Inc., paid for those services, Schwartz recalled.

His testimony came at the start of the sixth week of the government’s case, which charges cigarette makers conspired for 50 years to lie and confuse the public about the dangers of smoking.

The government suit, launched in 1999, targets Altria and its Philip Morris USA unit; Loews Corp.‘s Lorillard Tobacco unit, which has a tracking stock, Carolina Group; Vector Group Ltd.‘s Liggett Group; Reynolds American Inc.‘s RJ Reynolds Tobacco unit and British American Tobacco Plc unit British American Tobacco Investments Ltd.

The companies deny the allegations and say they have drastically changed their marketing practices since 1998, when they signed a landmark settlement with U.S. states that severely restricts marketing and subjects cigarette makers to oversight.

On the witness stand on Monday, Schwartz blamed the advocacy pressure on the Tobacco Institute’s public relations team, “who felt that we were not being cooperative enough.”

Government lawyers charged Schwartz’s ties to tobacco companies made him a favorable witness for the industry, but he defended his work as independent and “first rate.”

“It was, in fact, scientifically very productive ... and very satisfying,” Schwartz said.