A jury to be selected Monday in U.S. District Court in Oklahoma City could become the first in America to render a verdict on whether smokeless tobacco kills.

The jury will try a product-liability lawsuit based on the death from mouth cancer of Sean Marsee, a star high-school athlete in Ada, Okla. He began to use snuff when he was 12, getting a nicotine jolt by holding it in his mouth. About six years later, in 1983, he had three operations within six months and was badly disfigured. He died in February 1984, after physicians deemed a fourth round of surgery futile. He was 19.

Sean was the oldest of the five children of Betty Marsee, a widow and registered nurse who is asking damages of $147.5 million from U.S. Tobacco Co., maker of Copenhagen and Skoal snuff, the world's two best-selling brands of moist smokeless tobacco.

The Marsee case differs from the recent wave of cigarette lawsuits. No smoker has yet won, or is currently within striking distance of a court victory, in large part because of a powerful argument deployed by the manufacturers: For two decades, they have printed on every pack and in every ad the warnings that Congress required.

Marsee's lawyers, the husband-and-wife team of George and Dania Deschamps-Braly, do not have to face that argument because the government did not legislate smokeless-tobacco warnings until two years after Sean's death.

Alleging wrongful conduct, principally a failure to warn of known hazards, Marsee wants punitive damages of $136.5 million. This was the net income of the Greenwich, Conn., company in 1983, the year in which Sean fell gravely ill. She also seeks a compensatory award of $11 million.

Last year, U.S. Tobacco sold 480.8 million cans of its several brands of moist smokeless tobacco in this country alone -- up 17.3 million from 1984 and 54.9 million from 1983. Tobacco accounted for 86 percent of net sales of $480 million.

The Marsee case, which has drawn wide media attention, including a "60 Minutes" segment and a Reader's Digest article, prompted Congress in February to regulate smokeless tobacco for the first time. The new law requires a rotating series of strong warnings on every container and in every print advertisement, and bans television and radio ads after August.

For the six jurors who will try the case over the next several weeks, the pivotal issue is relatively narrow: whether one youth's use of two cans of Copenhagen every three days caused his fatal oral cancer. But the issue has broad, long-term implications for the public health, as well as for the $1 billion-a-year smokeless-tobacco industry.

U.S. Surgeon General C. Everett Koop has estimated that at least 12 million Americans -- mostly male adolescents and young men -- use about 165 brands of moist and dry snuff and chewing tobacco, and 6 million of them use it at least once a week. Koop said the numbers are increasing, and U.S. Tobacco's rising sales figures support him.

It was on the basis of medical research and testimony that Congress mandated warnings that smokeless-tobacco products "may cause oral cancer," "may cause gum disease and tooth loss," and -- contrary to the reported belief of Sean Marsee -- "are not a safe alternative to cigarettes." Oral cancer annually strikes about 29,000 Americans and kills about 9,000 of them.

Similarly, a surgeon general's advisory committee concluded in an April report: " . . . The oral use of smokeless tobacco represents a significant health risk. It is not a safe substitute for smoking cigarettes. It can cause cancer and a number of noncancerous oral conditions and can lead to nicotine addiction and dependence. . . . The scientific evidence is strong that the use of snuff can cause cancer in humans. . . . The excess risk of cancer of the cheek and gum may reach nearly fiftyfold among long-term snuff users."

U.S. Tobacco's top officers assured stockholders in a February letter that "an objective view of the scientific evidence" does not warrant such conclusions. "We believe that it has not been scientifically established that smokeless tobacco is a cause of any human disease, including oral cancer," wrote Louis F. Bantle, chairman and chief executive officer, and President Nicholas A. Buoniconti, the former Miami Dolphins linebacker.

Two substances that cause cancer in lab animals -- compounds in a class of potent carcinogens called nitrosamines -- can form in a chemical reaction between tobacco and saliva.

The surgeon general's committee said that nitrosamines "often have been detected at levels 100 or more times higher than government-regulated levels of other nitrosamines permitted in foods," and that the scientific evidence "strongly supports the epidemiological finding that use of smokeless tobacco causes cancer in humans."

Another issue is whether U.S. Tobacco promoted tobacco to the young. "The highest rates of use are seen among teen-age and young adult males," the advisory panel said. A Senate report cited "alarming use by children."

It "is not true" that the company has promoted its products to youth, Bantle told the stockholders. "For decades, U.S. Tobacco has strictly adhered to policies prohibiting such practices. . . . We have always considered smokeless tobacco to be an adult product and have marketed it as such."

Bantle had a different emphasis in a 1977 interview in which he told the Chicago Tribune: "We've gotten excellent sales growth from young people. . . . In Texas today, a kid doesn't dare go to school, even if he doesn't use the product, without a can in his Levis."

In annual reports, the company has stressed its financing of "strategically important" promotions aimed at "active, outdoor-oriented adult males" and providing "opportunities to give samples of our products to millions of potential customers," including auto-racing circuits, college rodeos and rock concerts. It spent $1 million on TV ads during the 1984 Winter Olympics.

Some critics charge that the promotions actually target youths as well as adults. One of the critics is Dr. Gregory N. Connolly, director of dental health in the Massachusetts Department of Health. His successful campaign for tough state regulation finally led the industry to join with health groups -- its natural foes -- in seeking a uniform federal law.

"I allege that U.S. Tobacco's promotional efforts in the Northeast contributed directly to our findings that 28 percent of our male high school students in Massachusetts tried smokeless tobacco in 1984, and that 12 percent were frequent users," Connolly said in an interview. "The company did this through use of sports figures . . . and distribution of free samples."

Connolly said a verdict for Marsee would lead to the filing of many new lawsuits and bring the company's sales growth "to a screeching halt." A verdict for the company would "make it extremely difficult for anyone else to sue," he said.