BOSTON--Discoveries reported in recent days are peeling away the mystery from Alzheimer's disease and pointing toward the cause and perhaps the cure of the leading cause of senility among elderly Americans.

"We are at an exciting time in this research," said Dr. Andrew Monjan, acting chief of the neuroscience of aging branch of the National Institute on Aging. Potential causes and cures "are no longer as distant," he said.

Three new pieces of work coming out last week were exciting, agreed Dr. Katherine Bick, deputy director of the National Institute for Neurological and Communicative Disorders and Stroke.

"The data show that we haven't been barking up the wrong trees in a number of the areas that are being dealt with," she said. "But I don't think this is the last word on any of these things. We've just moved a step forward and now are going to be able to ask a few more sophisticated questions."

Alzheimer's disease destroys elderly people's memory, their speech, their mobility and even their ability to feed themselves. An estimated 2.5 million Americans have it, and 100,000 die from it each year.

Despite its prevalence, however, Alzheimer's disease is tough to recognize. Until now, there has been no clear hallmark of the disease. Diagnosing it amounts to ruling out other illnesses that produce similar symptoms.

At a scientific meeting last week in Washington, Dr. Peter Davies of Albert Einstein College of Medicine in New York told of tracking down a protein called A-68.

If this work holds up to further scrutiny, it could be the basis of the first test for Alzheimer's.

It appears as though large amounts of this protein occur in the brains of Alzheimer's victims but not those of healthy people. This means it may be possible to diagnose the disease in its early stages with an ordinary spinal tap. Such early diagnosis will probably be essential if medicine is to be successful in stopping the disease before it destroys the brain.

Finding such a test has been a top goal of government-sponsored research, Monjan said.

"If we are able to come up with a treatment," he said in an interview, "the earlier we can intervene in the development of the disease, the greater the likelihood that we can make an impact on the outcome of the disease."

No such cure exists for Alzheimer's disease, and none is imminent. But another new study suggests a possible way to at least relieve some of the worst symptoms. Dr. William K. Summers of the University of California, Los Angeles, last week published his experiments with a long-forgotten drug that seems, at least temporarily, to sharpen victims' memories.

While taking this drug, patients were able to help care for themselves, recognize relatives, and in less severe cases, even play golf or work part-time.

Summers is the only researcher to achieve success so far with this experimental drug, called tetrahydroaminoacrine, or THA. Monjan said the medicine will have to be subjected to large, well-controlled studies before experts can be certain it is truly effective.

Another thread of evidence revealed last week was the isolation of the gene responsible for a protein that may play a key role in Alzheimer's. This gene, found by Dr. Dmitry V. Goldhaber and colleagues from the National Institutes of Health, makes a protein that clogs the brains of people with the disease.

Learning about this gene and the protein it makes could help scientists understand the underlying mechanisms that go awry when Alzheimer's strikes. Such basic insights should be valuable for tailoring a treatment or cure for the disease.

"Like most good pieces of research," said Bick, "it raises more questions than it answers."

That three such important development could come to light in the same week, Monjan said, "reflects the pace of the research and maybe even the rate at which the disease is becoming elucidated."