Harry Mills, the baritone member of the singing Mills Brothers, a smoothly melodious group that helped popularize hundreds of American hit songs through more than five decades of performing, died yesterday at his home in Los Angeles. He was 68.

Mr. Mills and his two brothers, Herbert, 70, and Donald, 67, started singing together as youngsters in their home town of Piqua, Ohio, and last performed as a group in September in Atlantic City. In between, they made more than 1,200 recordings and worked extensively in radio, television and motion pictures.

Two weeks after the brothers gave their final performance, Mr. Mills underwent surgery for the removal of a tumor. A diabetic, he had been bedridden for several months before his death.

For many years a quartet that included first another brother and later their father, the Mills Brothers were known for the style, harmony and showmanship of their renditions of such hits as "Tiger Rag," "Paper Doll," "Glow Worm," "Bye-Bye Blackbird" and "Up a Lazy River."

"I don't know why our songs are so popular now," Mr. Mills said in 1969. "Maybe people want to hear plain, simple songs and easy harmony."

The brothers performed here in 1976 on a Smithsonian Institution program and were described at the time as an institution in themselves.

"Their contributions have been amazing," a Smithsonian official said. "They were into so many things and at the beginning of so many trends. Nearly all the groups of the '40s and '50s until now owe something to the Mills Brothers."

One of the group's hallmarks was its vocal mimicking of musical instruments. That began, Mr. Mills once said, when they were youngsters and heard band broadcasts on the radio. "That's how we wanted to sound," he recalled, "like a band." Using kazoos to imitate horns, they called themselves Four Boys and A Kazoo.

One night, however, Mr. Mills lost his kazoo on stage and he continued performing without it. "That's the story of the Mills Brothers," he said. "We threw away those kazoos and started imitating different horns with just our throats and voices."

After performing live and on the radio in Cincinnati, they were discovered through a process that included singing over the telephone for an agent in New York. A network radio contract led to national fame. In 1934 they gave a command performance for King George V of England.

The eldest brother, John Jr. died in 1936, and their father joined them for the next 20 years. After his retirement, the act became a trio.