An estimated 2,500 Americans who received blood transfusions from donors who later tested positive for the acquired immune deficiency syndrome virus will receive letters from their physicians in the next few months urging them to have a blood test for the virus, the American Red Cross announced yesterday.

Under the "Look Back" Program, the Red Cross said it will alert blood banks, hospitals and physicians of patients who received blood from donors who later showed the presence of antibodies of the HTLV-3 virus -- the virus that causes AIDS.

Not everyone who tests positive for the antibodies will get AIDS. About 5 percent to 10 percent will develop AIDS, which has no known cure. But another 25 percent will develop AIDS-related complex, a lesser form of the disease.

The program is designed to "help curtail the further spread of AIDS by alerting those patients who may have received blood before the initiation of the HTLV-3 screening tests," a Red Cross spokesman said.

Stringent testing of donated blood, first begun on March 2, 1985, now protects the blood supply from contamination. When a blood donor gives a pint blood of today, the Red Cross knows within hours whether it is positive for the HTLV-3 virus. But prior to March 1985, there was no way for blood collecting organizations such as the Red Cross to determine which blood donors had been exposed to the AIDS virus.

Based on a year-long screening of recent blood donors, the Red Cross has uncovered 1,600 people who donated blood and then tested positive for the HTLV-virus, reported Dr. S. Gerald Sandler, associate vice president, medical operations for National Red Cross. All of the blood these people donated was discarded.

But some of the blood donors who tested positive for HTLV-3 had given blood in previous years. The "Look Back" program is designed to track -- and test -- the patients who received blood which Red Cross officials think could have been contaminated.

"We have made the assumption that an ill person who is being transfused with blood from a person with this HTLV-3 infection, will turn up positive in a blood test for the HTLV-3 virus," Sandler said.

The American Red Cross collects 6 million units of blood each year in the United States -- half of all blood collected in the country. Some 3.4 million Americans receive pints of blood collected by the Red Cross.

The other half of the blood supply is administered by the American Association of Blood Banks and by the Council of Community Blood Centers.

Negotiations are under way between the Red Cross and these organizations to coordinate a similar "Look Back" initiative, Sandler said.

"I can't comment at all on what the other half of the blood collecting organizations will do," he said. "But I expect that they will take the same initiative.

"The American Red Cross obviously regrets any emotional stress this notification process may generate," Sandler said. "We have voluntarily undertaken this initiative not to alarm the public but in the best Red Cross tradition of responsible service to the nation."

In a related development, researchers at the National Institute of Drug Abuse found that 17 of 1,129 stored blood samples taken from drug users in the United States in 1971 and 1972 tested positively for the AIDS virus -- an indication that the deadly virus may have been present in the United States far longer than researchers had believed.