Melvyn Douglas, 80, an actor who won enduring success in stage, screen and television productions that ranged from bedroom farce to high drama, died Tuesday at the Memorial Hospital of the Sloan-Kettering Cancer Center in New York City.He had pneumonia and a heart ailment.

Mr. Douglas won two Academy Awards as best supporting actor, for his performance as an elderly and tragic rancher in "Hud" in 1963 and in 1980 for "Being There," which starred Peter Sellers; a Tony Award, for his performance in the Broadway production of "The Best Man" in 1963; and an Emmy Award for his role in the 1968 television play about aging, "Do Not Go Gently Into That Good Night."

These honors came in the second half of his career, when he often was cast as an elderly man, as befitted his years and his skill. Sometimes he was crotchety, sometimes lovable and sometimes puckish. He was always in character and many of his characters were memorable. A notable example was his portrayal of a crusty old man in "I Never Sang For My Father," for which he was nominated for an Academy Award in 1970.

In the first half of his career, which began in 1919 and continued until his death, Mr. Douglas was a leading ladies' man in a series of comedies, spoofs and light-hearted mysteries in which he played opposite most of the leading actresses of the 1930s and 1940s, including Rosalind Russell, Claudette Colbert, Katherine Hepburn, Gloria Swanson and Greta Garbo. After starring with Garbo in "Ninotchka," which appeared in 1939, he became known as "the man who made Garbo laugh."

"Dapper and invariably frivolous, he chased after, or was chased by, all the comediennes of the time," wrote David Shipman in "The Great Movie Stars: The Golden Years." "He was a prop for the girls, unlike, say, William powell, who was a star in his own right."

Mr. Douglas himself once said, "I enarned what became an international reputation for being one of the most debonair and witty farceurs in Hollywood." On another occasion he said, "I enjoyed Hollywood until after the war. Then it began to bother me. I think it was the shallow roles offered me that caused my unhappiness."

Although the parts he got in the earlier years were similar, he resisted type-casting and the notion in later years that he was a "character" actor.

"All acting, if it's any good, is character acting," he said.

Apart from his work on stage and screen, Mr. Douglas and his wife, Helen Gahagan Douglas, who died last year, supported numerous liberal causes and sometimes were denounced as "reds." During the Spanish Civil War, Mr. Douglas was outspoken in behalf of the Loyalists and against the fascists led by Gen. Francisco Franco. He was a member of the California State Board of Welfare, a delegate to the 1940 Democratic National Convention, an organizer of the California White House Conference on Children and active in veterans groups.

Mrs. Douglas, a former singer and actress, served three terms as a Democratic congresswoman from California. She was defeated by Richard M. Nixon for a seat in the U.S. Senate in 1950. In the campaign, Nixon repeatedly suggested that she was a communist sympathizer.

"I have never regretted the stands I took," Mr. Douglas said. "I've always been a strongly anti-communist liberal."

Melvyn Edouard Hesselberg was born on April 5, 1901, at Macon, Ga. His father was a Russian-born pianist. His mother, Lena Shackelford, was of Scottish descent. The boy grew up in Macon, in Nashville and in Toronto. His father, Edouard, wanted him to be a musician. His mother wanted him to be a lawyer.

Young Melvyn tried to enlist in the Canadian Army during World War I. When this proved unsuccessful because he was only 13, he enlisted in the U.S Army at the age of 15 and served at Fort Lewis, Wash. After his discharge, he went home and announced that he would be an actor, having enjoyed appearances in high school productions.His parents were so opposed to this that he left and went to Chicago. There he studied acting -- one of his fellow students was Ralph Bellamy -- and worked for the City News Bureau as a reporter. He was fired from that job for going to sleep during a big story.

His first stage appearances was in Shakespeare's "The Merchant of Venice, which was put on by the company of William Owen, his drama coach. About that time he changed his name from Melvyn Hesselberg to Melvyn Douglas, choosing to be Douglas because of his mother's Scottish heritage. He spent the next several years touring the Middle West with stock companies.

Mr. Douglas' Broadway debut was as a gangster in "A Free Soul" in 1928. He appeared in several more Broadway productions between 1928 and 1931.In the latter, year, he starred with Helen Gahagan in "Tonight or Never," the last play to be produced by the legendary David Belasco. Not only was it a critical success for Mr. Douglas, but he and Miss Gahagan, whom Heywood Broun described as "one of the 10 most beautiful women in the world," fell in love and were married during the play's run. It was Mr. Douglas' second marriage.

"Tonight or Never" led to Mr. Douglas' first Hollywood role. He went west to make the film version of the play, which starred Gloria Swanson. This started him on the long series of comedies and romances that kept him busy until the United States entered World War II.

During the war, Mr. Douglas enlisted in the Army, was commissioned and rose to the rank of major. His duties involved putting on shows for sevicemen.

After the war, he appeared in "Sea of Grass," which appeared in 1947 and starred Katherine Hepburn and Spencer Tracy, "The Guilt of Janet Ames," with Rosalind Russell, and Mr. Blandings Builds His Dream House," with Cary Grant and Myrna Loy. In 1949, he returned to Broadway to appear in "Two Blind Mice." From then until his death, Mr. Douglas divided his time between Hollywood, Broadway and television.

His 70th film, "Tell Me a Riddle," was released this spring. Shortly before he died, he completed filming a thriller, "Ghost Story," with Fred Astair and Douglas Fairbanks Jr.

Mr. Douglas's survivors include two sons, Peter of New York City, and Gregory, of Worcester, Mass., one daughter, Mary Helen Douglas of Fairlee, Vt.; Six grandchildren, and one great-grandchild.

The family suggests that expressions of sympathy be in the form of contributions to the Sloan-Kettering Cancer Institute of New York.