BOSTON, FEB. 12 -- By infecting monkeys with a cousin of the AIDS virus, scientists have begun to overcome major barriers in their efforts to develop effective treatments and a vaccine to block the deadly disease.

When they are infected with Simian Immunodeficiency Virus (SIV), rhesus monkeys become ill with a disease that is strikingly similar to AIDS. SIV was first isolated from macaque monkeys in 1984.

Until recently, only chimpanzees were used in most tests of potential AIDS vaccines. But chimps are rare, cost more than $50,000 each, and the AIDS virus never makes them sick. The monkeys, on the other hand, are plentiful and easy to raise. They have begun to be used as the main research tool for testing potential vaccines and drug treatments.

"The effects {of SIV} are almost identical in every way to AIDS in man," said Dr. Norman L. Letvin of Harvard Medical School. "They develop the same opportunistic infections. Their immune systems become depleted, and many show signs of brain disorders."

Letvin made his remarks at the annual meeting of the American Association for the Advancement of Science here.

Although the research with monkeys will help answer many questions about how the AIDS virus invades the human body's immune system, it will still be years, most experts say, before an effective vaccine is developed.

Still, SIV is identical in many ways to HIV-1, the main human AIDS virus. Its genetic arrangements are similar, and because SIV causes a disease just like AIDS in monkeys, scientists will be able to try new drugs on the animals before the drugs are given to humans.

Most of the drugs under development for the treatment of AIDS are extremely powerful. Many have dangerous side effects. AZT, the only drug that has proved to be effective, causes severe anemia in almost half of those who take it.

"Tests on the monkeys will be a real boon to drug development, and that is such an important problem for us," said Dani Bolognesi, a leading AIDS researcher at Duke Universty Medical School. "It's the best model we have for this disease. But HIV and SIV are not the same virus, and nobody should forget that."

The human AIDS virus evolves quickly, and researchers say that means that it will not be possible to assume that what works in monkeys will always work in man. Because genetic mutations lead to modifications of the virus' molecular structure, it is possible that those changes could affect the behavior of the virus.

Nevertheless, the monkey model allows scientists to explore a series of puzzles arising from the unique behavior of HIV.

"We ask 'Why does the virus lay dormant so long?' and whether or not there could be a genetic component to who gets ill and how quickly," Letvin said. "This will allow us to systematically explore those issues."

He and other researchers have already begun to examine the relationship between low T-cell counts and progression of the disease. T-cells help the body fight disease; the AIDS virus kills them.

But by infecting the monkeys and mixing their T-cells with those of healthy monkeys in a laboratory dish, scientists can gauge how important each cell component is in protecting the body against disease.

In this systematic way, Letvin and other researchers hope to gain a much more complete picture of what role each element of the AIDS virus plays in infection and illness.

Eventually, the researchers will have to return to chimps. They are the only animals that can be infected with the human AIDS virus because they are so similar to humans.

"Monkey trials will give us the chance for trial and error that we need," Letvin said. "Without trial and error, science rarely accomplishes much."