ATLANTA (AP) _ A two-year international study of more than 17,000 heart patients showed that aspirin and the clot-dissolving drug streptokinase, taken together after the onset of chest pains, reduce deaths among heart attack victims.

The Second International Study on Infarct Survival, involving patients in 400 hospitals in 16 countries, is billed as the largest heart attack treatment study ever. It was presented Tuesday at the annual meeting of the American College of Cardiology.

The study showed that a chewed aspirin tablet or administration of the drug streptokinase, or SK, improved the survival rate. The rate improved more, however, when the two were taken together.

Mortality among heart attack victims given the combination was 7.8 percent after five weeks, compared with 12.8 percent for patients given a placebo.

Drs. Rory Collins and Peter Sleight and researcher Richard Peto, all of the University of Oxford, England, said the improvements were evident across the range of patients, including high-risk cases involving stroke or patients older than 70.

The importance of the study, they said, lies more in the size of the sample than in what was done. They said other studies were too small to give a true picture of the effects of the drugs.

The median age of the patients was just over 60 years, and 23 percent were women. The study was funded by the British Heart Foundation and one maker of SK. A drug company donated the aspirin.

Peto said the study was done in hospitals in medically sophisticated countries, ''but an aspirin tablet will work in rural China.''

Sleight said the combination of drugs significantly reduced the likelihood of a second heart attack while the patient was hospitalized and reduced the likelihood of a stroke by about one-third.

Of 17,189 patients in the survey, 534 given placebos for both aspirin and SK died within five weeks. Of those given aspirin but a placebo in place of SK, 447 died. Of patients given SK and an aspirin placebo, 420 died. Of patients given both medications, 327 died. The choice of treatment was made at random.

Collins said some doctors were uncomfortable giving a placebo. But he said doctors were asked to expose a patient to a possible placebo only if they were uncertain of treatment for an individual case. They were not asked to reject other treatments if they were obviously called for, he said.

The sooner the drugs were administered, the better the results, the researchers said. But they reported significant improvement even when the drugs were administered 24 hours after the onset of pains.

It has been established that aspirin is of value in preventing heart attacks and for patients who have recovered from one, Sleight said.

''This plugs the gap in the middle,'' he said, saying the tests show it can help for patients who have just suffered an attack.

For patients with a history of heart problems, chewing an aspirin ''would be a very reasonable thing to do'' while waiting for help, he said.

Peto likened SK to a household drain cleaner that is administered once to dissolve obstructions in the blood flow to the heart.

It was developed in the 1950s and ''inadequately tested'' on small samples for years, Peto said. Its wider use as a heart attack treatment began stirring renewed interest several years ago.

It has just recently lost its patent protection and thus can be made by anybody, he said. The lack of a patent lowers the cost to about $200 per dose, a tenth of the cost of similar competitive drugs.