The first jury to try a lawsuit alleging that smokeless tobacco causes fatal mouth cancer heard sharply conflicting opening statements today.

The jurors -- four women and two men, plus three alternates -- were told by Dania Deschamps-Braly that Sean Marsee's regular use of United States Tobacco Co.'s Copenhagen brand of snuff for about six years caused him to die an excruciatingly slow and painful death at 19.

But the jurors also heard Alston Jennings argue for U.S. Tobacco that the company is blameless. Sean died of tongue cancer, and this particular disease is of unknown origin when it occurs in young people, he said.

The product-liability trial, which U.S. District Judge David L. Russell said he expects to last up to six weeks, will pit medical and scientific experts from the United States, India and other countries against each other.

The trial has certain David-and-Goliath aspects. Sean's mother, Betty Ann, is a widowed registered nurse of modest means who has raised four other children. Representing her is a husband-and-wife team from her home town of Ada, Okla., Deschamps-Braly and her husband, George W. Braly.

Betty Marsee seeks $147.5 million, mostly in punitive damages, from U.S. Tobacco, which in 1985 had the highest ratio (19.5 percent) of net earnings to net sales of any Fortune 500 corporation.

The company is represented by large law firms from Little Rock, Ark., New York and Oklahoma City.

Sports also figures prominently in the case. Sean was a high school track star who had been accepted for training to become an Army Ranger. The company has used numerous celebrity athletes to promote its snuff, and Jennings introduced to the jury Walt Garrison, a former pro football player and the company's southwest region vice president, who was sitting at the defense counsel's table.

Deschamps-Braly told the jury the evidence will show that U.S. Tobacco acted in "reckless disregard for the public safety." The company, she contended, promoted Copenhagen and other brands of snuff to youths and as a safe alternative to smoking, failed to warn snuff users of evidence dating back to 1915 that it may cause oral cancer, and could have, but did not, make a less hazardous product.

For the company, Jennings said his evidence will show that "Sean Marsee's tongue cancer was not caused by snuff," that the overwhelming majority of young snuff users do not get the disease, and that some youths who have never used snuff do get tongue cancer.

If the jurors agree with the company's position that snuff didn't cause his death, Jennings told them, the other issues raised by plaintiff's counsel will disappear from the case "because they have no consequence."

Braly-Deschamps said that extremely potent carcinogens called nitrosamines are present in U.S. Tobacco's snuff brands at levels "literally thousands of times higher" than in any other consumer product, and also at levels much higher than in rival brands.

She also said company records show that it knew of high nitrosamine levels in 1974 and that U.S. Tobacco's own research confirmed this in 1975.

Jennings said that nitrosamines do not produce cancer in lab animals and that "there is no evidence that nitrosamines or their metabolites [products formed by metabolism] cause cancer in human beings."

An advisory committee to the U.S. Surgeon General said recently that the available scientific evidence "strongly supports the epidemiological finding that use of smokeless tobacco causes cancer in humans."

Deschamps-Braly said a 1968 document shows then-marketing vice president Louis F. Bantle saying, as she put it, that "the time was ripe to sell snuff to young people to create a fad." Bantle is now chairman and chief executive officer and received compensation of $1.1 million last year.

The lawyer also said that a 1972 U.S. Tobacco document shows an "intent to hook 'em [new users in the 15-to-35 age range] early."

Betty Marsee, the first witness, said she discovered Sean's use of snuff -- two cans every three days -- when he was 15.

She quoted Sean as saying that if snuff were hazardous, there would be warnings on the can, just as there were warnings on cigarette packs, and that professional athletes would not be advertising it. Congress voted only last February to require warnings for smokeless tobacco products.