The Chilean-contaminated-grapes incident of the past week demonstrated again how fragile the defenses of Western open societies are to terrorists and lunatics. The act of injecting two grapes with cyanide was simple but so diabolical that it touched just about every American who goes to the grocery store.

In most major terrorist incidents of recent years the press has been made much the whipping boy. In this case there were the normal worries: that publicity might inspire "copycat" behavior, that we were playing into the hands of terrorists who thrive on media manias.But nowhere I read and saw the story was it told sensationally. The press spotlighted the crisis-management steps being taken by the stores, the Chileans and the Food and Drug Administration as well as fruit importers and others who scornfully gulped the grapes and called the FDA ban on Chilean fruit outlandish.

- FOR A NEAR PARALLEL go back to the Tylenol poisonings of September 1982. The press got good marks for its evenhanded coverage of that case, in which seven persons died after taking cyanide-laced capsules of the pain reliever Extra-Strength Tylenol.

There were dozens of copycat crimes, largely adulteration of food. And Tylenol's manufacturer, Johnson & Johnson, pointed out that there were literally hundreds of so-called "Tylenol poisonings." But when the deaths were checked out they were found not Tylenol connected. The company chairman said he was "impressed with how few of those hit the papers."

Johnson & Johnson also benefited in the long run by the way it managed the crisis: its candor, its prudent recall policy and its development of more tamper-proof packaging, all of which regained public confidence for the product when it reappeared three months later.

- THE PRESS ABSOLUTELY has a responsibility to inform the public about threats to health and life in products tainted through terrorism or other means.

Sometimes it takes risks for doing so. The best case in point was the Thalidomide investigation by the Sunday Times of London.

In the early 1960s Thalidomide was responsible for gross birth defects, babies with deformed or missing arms and legs, born to women who had used the sleeping pill.

But Britain has strict contempt of court laws against publication of material that might prejudice legal proceedings.

Because claims against the pharmaceutical company were pending, there had been no public disclosure of the extent of the cases (totaling 450) or the children's plight more than a decade after the births, and the claims were being settled for a pittance.

The editor of the Sunday Times, Harold Evans, took the view the public was entitled to know how a dangerous drug came to be produced and whether new safety regulations were sufficient. He also wanted to win more money for the parents. The paper spent years investigating the drug and compiled massive evidence. Then it went to Herculean lengths to get a ban against publication of the expose lifted. It was not until after the case was heard in the House of Lords and the European Commission of Human Rights that the attorney general asked that the injunction be withdrawn. The investigative article, six newspaper pages long, appeared in June 1975, 15 years after most of the births.

- PRODUCT TERRORISM is often bizarre as well as scary.

In Japan in 1984 a group calling itself "The Man With 21 Faces" panicked Japanese consumers with a campaign of extortion. It began when the thugs kidnapped the president of a major Japanese confectionery company. They demanded a ransom of 300 million yen, just over a million dollars then. After the president escaped from a warehouse where he had been kept captive, the criminals sent threatening notes to the media and the police saying they would inject cyanide into his candy products. Later, they turned their threats on another candymaker, sending the press copies of each letter they sent to the company. As a result of the publicity, the company's products disappeared from the shelves.

If there is an overarching model of press response in cases of this kind I don't know it. The dilemma of how threats should be publicized is still very much with us in the grape scare and in the debate over whether passengers of the ill-fated Pan Am jumbo jet bombed over Scotland should have been warned of the telephoned threats against the flight.

In the Tylenol case, Dr. Walter Menninger, senior staff psychiatrist of the Menninger Foundation, simply told a press groping for guidelines, "It's how much you keep it in perspective. You have to do your best, but you must recognize it's a hazardous world out there."

- IN ANOTHER CONTAMINATION case, Mike Korologos, vice president of Evans Advertising and Public Relations, last fall found the press mostly neutral and accurate.

His company has the Norbest turkey account. Just before Thanksgiving, Norbest ordered the recall of 400,000 pounds of fresh whole "Family Tradition" brand turkeys from eight Eastern states. Spoiled giblets or necks had been found in some birds, though the company stressed that no illnesses resulted.

Evans sent out press releases and made some 300 telephone calls, at least 200 responding to media requests for information. Newspapers and news programs were correct and calm in their coverage, but the jabber on some radio talk shows was not, Korologos found. Some Eastern disc jockeys hoaked the story up, as when one led into a newscast by crying, "There's trouble in Turkeyville!"

Korologos says the Salt Lake media did a particularly thorough and accurate job, though the story was not a strong local one. The turkeys were produced in the Midwest and the recall affected only some Eastern states.