A $6 million bequest - the largest single amount of money ever donated to Howard University - has been willed to the school by the late Dr. Cliland B. Powell, former editor and publisher of Harlem's Amsterdam News and the first black physician to become an X-ray specialist.

The gift, which is to be used in the university's student scholarship and loan program, was announced by Powell's widow, Lena, in Briarcliff Manor, N.Y., a university spokesman said yesterday.

The money will not be available for use by Howard until after Lena Powell's death. In addition, recipients of loans or scholarships must study medicine, pharmacy, dentistry or engineering, the spokesman said.

Dr. James E. Cheek, Howard University president, called the bequest a step forward in the school's $100 million capital fund drive. "This is a major moment in the life of the university," he said, "not only because the sum is so generous, but because the bequest came from a man revered by so many."

A native of Suffolk, Va., Powell, who died last September at age 83, graduated from Virginia Normal School in Petersburg, Va. In 1920, he graduated from the Howard University School of Medicine.

Powell amassed a fortune through his practice of medicine and as the founder and head of several black-oriented businesses.

As a physician, Powell was the first black to become an X-ray specialist and operated his own laboratory in Harlem for 25 years. He was an intern at New York's Harlem and Bellevue hospitals for eight years before opening his laboratory.

Powell and another physician, Dr. P. N. H. Savory, purchased the Amsterdam News in 1936. The newspaper eventually became a national forum for black equality during the civil rights movement of the 1960s. In 1971, Powell, who was then the sole owner, sold the paper to a group of black leaders.

Powell also founded and operated the Victory Mutual Life Insurance Co., the Community Personal Finance Corp., Midway Technical School and Unity Funeral Parlors.