Conductor Andre Kostelanetz, who brought classical music to mass audiences while helping the works of modern composers find their way into the classical repertoire, died Sunday night in a hospital in Port-au-Prince, Haiti. He was 78.

Mr. Kostelanetz's agent in New York said the Maestro had contracted pneumonia while vacationing at the resort of Habitation LeClerc. Mr. Kostelanetz has used a heart pacemaker for several years.

He was one of the best know conductors of his time. He first gained national fame conducting classical programs over the Columbia Broadcasting System in the 1930s. His recordings sold more than 52 million copies, a record.

He became a guest conductor of the New York Philharmonic in 1952 and was scheduled to lead that orchestra again on Feb. 9.

He initiated the Philharmonic's enormously popular summer Promenade series, which were played in Central Park from 1963 until they were ended because of rising production costs in 1978. The audiences sat at tables, where they could drink wine, and there were instrumentalists, mimes, dancers, narrators and puppeteers in addition to a program of light classical music. A black market developed for tickets to the concerts.

Last summer, Mr. Kostelanetz led the Philharmonic in a Central Park concert featuring the works of Stravinsky and Rimsky-Korsakoff. The event attracted an estimated 250,000 people, believed to be a record for a concert of classical music.

Although Mr. Kostelanetz drew much of the music he played from such 19th century romantic masters as Brahms and Tchaikovsky, he was a major force in bringing the work of contemporary composers to public attention. Much of their work now is regularly played by orchestras around the world.

Mr. Kostelanetz gave performances of works by Richard Rodgers, Alan Hovhaness, William Schuman, Virgil Thompson, Paul Creston, Aaron Copland, Cole Porter and George Gershwin, among others.

Moreover, he commissioned many pieces. Among them are Copland's widely popular "A Lincoln Portrait," Ferde Grofe's "Hudson River Suite," William Schuman's "New England Tryptch," and Jerome Kern's "Mark Twain."

At various times, Mr. Kostelanetz was characterized as more of a popularizer than a master of music, but as the years passed his musicianship came into very high regard.

He was a guest conductor at all the major symphonies in the United States, including the National Symphony in Washington and also appeared with the major orchestras of Euope, Israel and Japan.

He never denied the notion that he was seeking more people to hear his music.

"If I can leave an inheritance of a growing audience for the concert hall, I have accomplished everything," he once said. "All the rest are second-class accomplishments."

His last public appearance was as conductor of the San Francisco Symphony for four concerts during the Christmas season.

Mr. Kostelanetz was born in St. Petersburg, Russia, now Leningrad. His parents were ameteur musicians and he was educated at the St. Petersburg Conservatory. A story is told which says that he first conducted at the age of 5. His parents had taken him to a band concert in a park in St. Petersburg. The boy began conducting along with the band leader. The band leader stopped to watch, the boy kept conducting, and the band kept playing.

However that may be, Mr. Kostelanetz trained to be a conductor. He left the Soviet Union in 1922 and came to the United States. He was hired as a rehearsal accompanist with the Metropolitian Opera. In 1930, he was appointed conductor of the CBS Symphony. His life work of bringing more music to more people had begun.

In 1936, he made 13 consecutive weekend flights from New York to Hollywood to direct Lili Pons, the great coloratura soprano, in the musical "That Girl from Paris." Two years later, they were married.

Amond the finest recordings Mr. Kostelanetz made were some in which he directed Miss Pons in concert.

The Maestro, as he liked to be called, and Miss Pons honeymooned in South American. During that sojourn he conducted the Radio El Mundo Orchestra in a concert that was broadcast via shortwave to the United States and carried by CBS. He also conducted at the Colon Opera in Buenos Aires with Miss Pons appearing as soloist.

During World War II, Mr. Kostelanetz formed orchestras of soldiers in North Africa, Italy and the Middle East. For this the short and round-faced Mr. Kostelanetz was decorated by the governments of the United States and Canada.

In 1958, Mr. Kostelanetz and Miss Pons were divorced. In 1960, he married Sara Gene Orcutt. They were divorced several years later. He had no children.

Survivors include two sisters, Mrs. Marion Frank and Mrs. Alex Afan, and a brother, Boris Kostelanetz of New York City, where the Maestro also lived.