The first case of AIDS-contaminated blood slipping past routine and widely used screening tests was reported last week when government researchers announced that two men were infected while receiving transfusions of blood deemed free of the disease.

Colorado health officials were alerted last November when blood samples collected from a 31-year-old blood donor were found to contain antibodies against the AIDS virus. Blood that he had donated in April and August had tested negative for presence of antibodies to the virus, known as HTLV-3.

Checking hospital records, officials tracked down four patients who received transfusions with blood from the man's earlier donations and tested them for antibodies to the AIDS virus. Of the four, the two who received blood in August tested positive for the AIDS antibody while the two who received transfusions in April were negative. None reported symptoms of the disease.

The donor told officials he had had only two homosexual encounters, one in 1974 and the other in May 1985. The AIDS virus is sometimes transferred during homosexual encounters, and officials speculated that he became infected in the second encounter but had not produced antibodies against the virus when he donated blood in August.

Researchers have long been aware that a "window" of two weeks to six months exists between exposure to the virus and the development of antibodies. In addition, a small number of persons never develop the antibodies but remain active carriers.

Of the two infected patients, one is a 60-year-old heterosexual who has been married for 30 years and denied using intravenous drugs or having extramarital affairs. Although the other, a 57-year-old homosexual, said he has had multiple sexual partners, infection stemmming from the transfusion is "more probable," officials said.

The cases are described in the latest Centers for Disease Control weekly report, which advises: "The current risk of transfusion-associated infection is small. Currently availible screening tests detect HTLV-III-LAV antibodies in the great majority of infected persons."

But at the same time, the report cautions that "since antibody may not be detectable in blood from donors with very recent infections," the safety of the blood supply requires thorough interviewing of potential donors to discourage those who are in high-risk AIDS groups -- male homosexuals and intravenous drug users.

Since 1977, more than 21,000 AIDS cases have been reported in the nation. More than half the victims have died.

Dr. Jeffrey Laurence, director of AIDS research for New York Hospital Medical Center, said: "Right now there is no test that will absolutely guarantee that blood is safe . . . This was bound to happen sooner or later."