WASHINGTON (AP) _ Bone marrow cells can become a ''reservoir of infection'' for AIDS that may spread the disease to other parts of the body, a new study shows.

Thomas M. Folks, head of a virology laboratory at the federal Centers for Disease Control in Atlanta, said Thursday the finding confirms that the AIDS virus is able to infect and reproduce within stem cells - immature cells that grow in the bone marrow and develop into components of the blood.

''This is probably a cell that helps set up the reservoir of infection and allows the disease to become chronic and linger for a long period of time,'' Folks said in a telephone interview.

Folks said he and a team of researchers used a new technique to purify stem cells from bone marrow extracted from a cadaver. The stem cells then were exposed to human immunodeficiency virus, or HIV, that causes AIDS, and then cultured in test tubes.

''After 40 or 50 days, the cells in the culture became packed with HIV virus,'' Folks said. ''The cells became bags of virus.''

The infected cells developed into macrophages, a type of cell that fights disease by engulfing and digesting bacteria or other infective agents.

Folks said for people infected with AIDS, macrophages may develop into ''bags of virus'' that actually can spread the infection to other parts of the body. --- House Leaders Back Gonzalez for Committee Chairmanship

WASHINGTON (AP) - Rep. Henry B. Gonzalez, D-Texas, has won the backing of House Speaker Jim Wright and other Democratic leaders for the chairmanship of the House Banking Committee.

The endorsement all but guarantees that the 72-year-old Gonzalez, a 27-year veteran of Congress from San Antonio, will take over the committee at a critical time: during the debate over a taxpayer bailout of the fund that insures the nation's savings institutions.

The current chairman, Rep. Fernand J. St Germain, D-R.I., lost his re- election bid Tuesday after his opponent made a campaign issue out of Justice Department allegations that St Germain accepted thousands of dollars in entertainment from savings industry lobbyists.

Wright, at a news conference, said Democratic leaders met Thursday afternoon and unanimously decided to support Gonzalez as St Germain's replacement when the House Democratic Caucus meets on Dec. 6.

Others at the leadership meeting were House Majority Leader Thomas S. Foley of Washington, Majority Whip Tony Coelho of California, Deputy Whip David E. Bonior of Michigan, Beryl Anthony Jr. of Arkansas, chairman of the Democratic Congressional Campaign Committee; Budget Committee Chairman William Gray of Pennsylvania and Democratic Caucus Secretary Mary Rose Oakar of Ohio.

Ms. Oakar is also a member of the Banking Committee and had been mentioned as a possible alternative to Gonzalez. --- President OKs Back Benefits for Man Who Joined WWII Navy at Age 12

WASHINGTON (AP) - A World War II veteran injured at Guadalcanal will be given the disability benefits he was denied by the Navy because he lied about his age when he enlisted at age 12, under a bill signed by President Reagan.

Reagan said Calvin L. Graham ''is a veteran of Navy and Marine Corps service who has honorably served his country.''

The act provides that dental injuries Graham suffered at the Battle of Guadalcanal will be considered a permanent service-connected disability incurred in the line of duty. It also provides for reimbursement for costs for treatment.

The measure also grants Graham a lump-sum payment of $4,917 in back pay for the period from Sept. 17, 1942, to April 5, 1943, when the Navy discovered his age and he was mustered out. He'll also get mustering out pay and interest.

Graham was aboard the USS South Dakota when shrapnel ripped through his mouth. He contended he lost all his teeth, but was denied mustering out benefits and was granted disability status for only one tooth.

The legislation gives him $18,000 for his mouth injury, in addition to the mustering out and back pay.