Grandmother was on to something. Garlic really is good for the heart.

According to laboratory tests of various extracts of the pungent bulbs, garlic contains a substance that can act as an anticoagulant, or blood thinner, to prevent blood clots. If such clots lodge in the arteries supplying heart muscle, they can cause heart attacks. In the brain, they can cause strokes.

Though the natural form of garlic has weak anti-clotting properties, scientists say that they have modified it chemically to produce a potent anti-clotting chemical that shows promise as a treatment for people at special risk of heart attacks or strokes.

The findings were presented earlier this month at the International Chemical Congress of Pacific Basin Societies here, which drew more than 3,700 chemists from 38 countries bordering the Pacific.

The new anticoagulant is made from a substance that is not present in a whole garlic clove but forms when garlic is crushed or cooked. The chemical is produced when two chemicals in separate cells of the bulb react to each other.

The resultant chemical, called allicin from the Latin word for garlic, allium, has long been known to have antibiotic properties. (During World War I, crushed garlic was pressed into wounds to prevent gangrene.) The anticoagulant made from allicin is called ajoene, from the Spanish for garlic, ajo.

Eric Block, of the State University of New York at Albany, who announced the results, said previous findings of garlic's blood-thinning properties were wrong and based on uncoordinated efforts.

Block said ajoene appears to be about as strong as aspirin in preventing blood clots. Many doctors recommend taking small amounts of aspirin daily to minimize the risk of forming clots.

Block said ajoene seems to work by an entirely different mechanism, however, which may allow more precise control of blood clotting than is possible with aspirin, which produces many effects in the body.

"Our work has been inspired by thousands of years of folk tradition about the medicinal value of garlic," Block said. "However, it is our conclusion that the anticoagulant value of ordinary garlic is low enough that you would have to eat huge quantities every day to get the same value as the compound we have synthesized from garlic.

"I think you'd lose your friends long before you got any medicinal value out of it."