The physician who treated and operated on Sean Marsee, who died of mouth cancer at the age of 19, told a jury today that the youth "would not have developed the cancer on his tongue if he had not used Copenhagen snuff for six years."

Dr. Carl S. Hook, a head and neck surgeon in Ada, Okla., was the second witness for Betty Ann Marsee, the youth's mother, in a product-liability trial. She is seeking punitive and compensatory damages from United States Tobacco Co., maker of Copenhagen, Skoal and other brands of snuff.

Hook testified that he knew of nothing but the smokeless tobacco, which Sean held in the right side of his mouth, that could account for the cancer that developed on the right side of his tongue. He emphasized that Sean, a high school track star in Ada, had been in "excellent health."

Hook told George W. Braly, a lawyer for Betty Marsee, that he based his conclusion on his own experience, on medical literature dating back to 1915, and on the views of other head and neck surgeons.

In cross-examination that will resume Wednesday, U.S. Tobacco attorney Alston Jennings presented several articles from authoritative medical journals that told of tongue cancer in boys and youths who had not used tobacco in any form. Hook agreed that some cases of unknown origin occur but insisted they are few.

One of the articles said that, of five boys in Western India who were stricken by tongue cancer between the ages of 11 and 17, only one had any history of tobacco use. The article appeared in 1976. Hook had told Braly that a sharp increase in tongue cancer among young snuff users occurred after 1975, when he began to practice.

A sharp dispute in the case centers on U.S. Tobacco's advertising and promotion of snuff starting in the late 1960s. Figuring in the dispute are depositions taken last month from Louis F. Bantle, chairman and chief executive officer, and Walt Garrison, a vice president and former linebacker for the Dallas Cowboys who has been sitting at the defense counsel table.

Although U.S. District Judge David L. Russell has yet to put the sworn statements into court records, several references to them appear in pretrial documents.

Bantle denied -- in annual reports to stockholders as well as in the deposition -- that U.S. Tobacco promoted snuff use to young people.

Attached to the deposition, according to a pretrial memo, are the minutes of a meeting in 1968 at which Bantle, then vice president for marketing, is quoted as saying: "We must sell the use of tobacco in the mouth and appeal to young people." Another document shows that the company let the Copenhagen and Skoal brand names be used on toy cars for 10-year-olds.

Bantle and Garrison denied that the company had distributed free snuff samples to minors, and Bantle said this reflected a policy dating back to 1936, according to pretrial memos.

Marsee's lawyers have listed 10 witnesses who they say will testify that as teen-agers they received free snuff samples from U.S. Tobacco. Betty Marsee claims her son started on snuff after receiving free samples at a rodeo.

A pretrial memo says that Bantle denied in his deposition that U.S. Tobacco ever had marketed to 15-year-olds. The minutes of a 1972 meeting that was attended by "nearly every important person in the company," according to Braly, describes the target audience for snuff as "new users, mainly cigarette smokers, age group 15 to 35."

Another internal company document in the pretrial files discusses whether to put a health warning on snuff cans.

"There is a benefit from the product-liability point of view in the warning notice," the document says. "We may have to put a warning on ourselves. Legally it is better if we are forced rather than voluntarily putting one on."

Braly said in a memo to Judge Russell that "this remarkable position . . . constitutes a new nadir in the list of all-time worst examples of complete corporate disregard for the safety of consumers of its products." He cited court decisions that manufacturers must warn consumers of scientific evidence "tending to show" hazards.

The company said, also in pretrial documents, that it considers a product "safe" if claims of adverse health effects have not been "proven."