John Donald MacArthur, 80, an insurance and real estate tycoon believed to be one of America's two remaining billionaires, died today at Good Samaritan Hospital here.

His physician had told a news conference three days ago that Mr. MacArthur was dying of cancer of the pancreas.

The man who rose from a childhood of poverty to become fabulously wealthy, always lived modestly, even frugally. His ambition was to reach 80, which he did last March 6.

"I never started out to make a great fortune. I never put a price tag on my efforts. I stayed with it," Mr. MacArthur said in a Canadian documentary film shortly before he suffered a stroke in the fall of 1976.

He never fully recovered from the stroke, and for awhile went to a speech therapist, but quit when he became frustrated at his rate of recovery. His right hand never regained full strength. His signature - used to conclude million dollar deals - had to be carefully drawn, letter by letter.

Most of the time, his mind was quick and witty. He insisted onworking until the time of his death and he maintained his reputation for being "salty-tongued" and "crusty" in reporters' descriptions of him.

Mr. MacArthur enjoyed being around reporters, having once been one for the Chicago Herald Examiner. He was dubbed "the accessible billionaire" by the Canadian documentary film.

Mr. MacArthur was Palm Beach County's largest landowner and one of the largest in the state of Florida. He owned 45 companies and exployed about 15,000 people.

But things weren't always like that for the eight-grade dropout. Born in Pittston, Pa., he was the son of a farmer who became a self-ordained minister. After dropping out of school, Mr. MacArthur served as his father's assistant for two years before moving to Chicago to work in his older brother Alfred's insurance company.

Within three months, he became the company's top salesman, but he was unhappy, and joined his brother Charles on the Chicago Herald Examiner. Charles went on to become a famous playwright and co-author of "The Front Page."

Charles' widow, actress Helen Hayes, yesterday called her brother-in-law "a tortured man who was uneasy with people." But she said his interviews were "brilliant" and he had "a gift for words."

After a short stint as a police reporter, Mr. MacArthur went back to work with his brother, Alfred. Then he went to Canada to become a pilot in the Royal Flying Corps, during World War I. Instead of going to battle, he was sent to Texas as a flight instructor. He went AWOL and ended up on a U.S. Army troopships to Europe. He managed to become a war hero.

Luck played an important part in his climb to the top. But there also was determination, drive, long hours and the willingness to take a chance, along with a convincing saldes pitch.

Mr. MacArthur married Louise Ingals. They had two children, Roderick John and Virginia. After their divorce, he married his brother Alfred's secretary, Catherine Hyland.

It was with his second wife that he began to build his financial empire. Together in 1928, they bought the Marquette Life Insurance Co. They struggled to keep it together during the Depression.

In 1935, he acquired Bankers Life for $2,500, and expanded it by selling $1 insurance policies through newspaper ads. If people had only $1 to spare, he was after that dollar. It was something that had never been tried before and the money poured in.

His practices came under sharp scrutiny. Between 1948 and 1951, Mr. MacArthur was investigated by 14 state insurance departments. He was the sole stockholder of Bankers Life until October, 1974, when he resigned as president and put all the shares into a tax-exempt charitable foundation. He became chairman of the board and chief executive officer. He named his wife and son, and veteran radio commentator Paul Harvey, a close friend, as foundation trustees.

Every March, Bankers Life has held a special observance during which salesman strive for their best month of sales in honor of the "Skipper's" birthday.

Bankers Life is just one of the 13 insurance companies Mr. McArthur owned. They included Bankers Multiple Life Insurance Co., Certified Life Insurance Co. of California, Constitution Life Insurance Co., Marquette Life Insurance Co., Protection Mutual Insurance Co. of Pennysylania, Southern Title and Insurance Co., Gotham Life Insurance Co. of New York, Union Bankers Insurance co., Western American Life Insurance Co. and Western Life Assurance Co. of Hamilton, Ohio.

Life insurance wasn't Mr. MacArthur's only interest. He owned about 30 other companies, 100,000 acres of land in Florida, land in Illinois, Arizona, Georgia, Colorade, Michigan and Wisconsin, hotels, golf courses, paper mills, farms, 61 buildings in New York City, utility firms, oil wells, real estate firms, restaurants, a car rental firm and a liquor firm that named a bottle of whiskey after him.

He owned several miles of undeveloped ocean front property in Palm Beach County and was the sole stockholder of Citizens Bank and Trust Co. in Illinois.

Owner of the J.D.M. Country Club in Palm Beach Gardens, Fla., he had developed the town in 1959, the year he came to Florida.

Mr. MacArthur had a 10,300-acre cattle and citrus rance in Highlands County where he would escape occasionally for relaxation; 32,000 acres in Sarasota County, and about 10,000 acre in the Orlando area.

He always lived simply. Until he moved to his Colonnades Beach Hotel on Singer Island in 1966, he and his wife had lived in a small home in Lake Park. His two-story apartment with an ocean view at the Colonnades was comfortably but modestly furnished.

He didn't like dressing up, and most of the time wore a sports shirt and slacks. When he wore a coat, it was usually his Bankers Life jacket, made from his family's green clan tartan.

Mr. MacArthur was 79 years old the first and only time he ever wore white tie and tails.The occasion was a televised state dinner at the White House for Queen Elizabeth of England.

He didn't have a fancy office. Most of his dealings were done at a corner table in the Colonnades coffee shop. Beside the table were two telephones he used constantly, one a national long distance line.

Behind his chair, a pot of coffee sat warming on a burner within reach. He drank more than 20 cups a day.

He had no fear of being kidnaped, brush off the idea of having bodyguards and drove himself wherever he wanted to go. The peppery billionaire prided himself on living up to the Scottish reputation for being frugal.

He always flew tourist class. On a flight to Chicago to celebrate his 80th birthday, he leaned over and asked the stranger next to him if he was going to eat his untouched piece of pecan pie.

Assured it was unwanted, Mr. MacArthur gingerly wrapped the pie in a napkin, looked to see if the stewardess was watching, and stuffed it into his travel bag.

He loved to tell people: "I'm Scotch."

In the 1970s, Mr. MacArthur liked to play lawyer and represent himself when called on the carpet by counties or municipal officials for zoning, building or health regulations.

He didn't want people to think him philanthropic, but he had many pet charities.

In the 1950s, he financed the production of a film, "America's Untapped Assets," which was made at the Bankers Life and Casualty Co. where one-quarter of the work force was handicapped.

The film was shown many times on television as a public service and he received a commendation from the President's Committee on employment of the Handicapped.

In 1965, Mr. MaxArthur supplied the $25,000 ransom money that resulted in the recovery of the stolen 103-carat DeLong ruby. He said he put up the money as a public service and a tax deduction.

One of his pet charities was the Animal Rescue League. Two silver, miniature poodles had the run of his apartment. The courtyard of the Colonnades is covered with ducks. He liked to watch them and feed them.

As a wealthy man, Mr. MacArthur was the target of numerous legal actions. Some of the cases were settled, despite his complicated business holdings. Some are still pending.

He is survived by his wife, Catherine: a son, Roderick, of Chicago: a daughter, Virginia Cordova, of Mexico City, and six grandchildren.