Ethel Carow Roosevelt Derby, 86, a daughter of President Theodore Roosevelt, died Saturday at her home, Old Adam House, in Oyster Bay, Long Island.

Mrs. Derby was a quiet Roosevelt. Unlike her half sister, Mrs. Alice Roosevelt Longworth, the grande dame of Washington, she shunned publicity, but she was neither retiring nor reticent.

She supported the proposed revision of the Panama Canal Treaty, although her father was the President who acquired control over the Canal Zone for the United States and who started construction of the waterway. She was a nurse in France during World War I and headed the Nassau County, L.I., chapter of the American Red Cross in World War II. She was honorary chairman of it at the time of her death.

She played a major role in preserving Sagamore Hill, her father's estate at Oyster Bay and having it placed on the National Trust for Historical Preservation. She was a member of the board of directors of the Museum of Natural History in New York.

"If there was any local activity of any kind, she either started it or was in it," said Leonard Hall, a former chairman of the Republican National Committee and a congressman whose district was made up of part of Nassau County.

"The lady who led the charge up Sagamore Hill was Ethel Derby," Hall said. "'No' was not in her vocabulary."

Hall recalled that some residents of the county were concerned about traffic congestion on the narrow roads of Oyster Bay caused by the number of visitors to Sagamore Hill. He said Mrs. Derby persuaded the county to build a road that would accommodate this traffic and thereby solve the problem.

Mrs. Derby herself sometimes used to conduct tours of Sagamore Hill, where she spent her childhood. "Do ask questions," she instructed two college students on one of her tours, according to a newspaper account. "I love to answer questions from young people. It is the only way you will learn anything."

On the question of the Panama Canal, Mrs. Derby favored the controversial proposal to replace the existing treaty. She quoted a poem by James Russell Lowell to support her point:

"New occasions teach new duties,

"Time makes ancient good unconth

="They must upward still and onward

Who keep abreast of truth."

On avoiding the public eye, she once told an interviewer, "I have an instinct for privacy." She added that she was "as reticent about publicity as my sister is comfortable with it." This was a reference to Mrs. Longworth, of whom Mrs. Derby was extremely fond.

Mrs. Derby was born at Sagamore Hill. Her mother, the former Edith Kermit Carow, first wife having died after the birth of Mrs. Longworth. Mrs. Derby was a first cousin of Eleanor Roosevelt and a distant cousin of Franklin Roosevelt. She made her debut into society from the White House and in 1913 married the late Dr. Richard Derby, a surgeon.

During World War I, she went to France with the Roosevelt Hospital Group as a nurse. When the United States joined the conflict in 1917, her husband also went to France as an Army doctor. When they returned from the war, they bought the Old Adam House in Oyster Bay, and Mrs. Derby lived in for the rest of her life except for summers in Cavendish, Vt.

In addition to Mrs. Longworth, survivors include two daughters, Edith Derby Williams, of Seattle, Wash., and Sarah Alden Gannett, of Brattlebor, Vt.; a brother, Archibald B. Roosevelt, of Hobe Sound, Fla., and nine grandchildren.

Three other brothers died in war. Thye were Quentin, a pilot who was shot down in World War I, Gen. Theodore Jr., who won the Medal of Honor during the Normandy invasion in World War II and died shortly thereafter, and Kermit, who died in Alaska during World War II.