The death of a New York woman who took a cyanide-tainted Extra-Strength Tylenol capsule was an isolated homicide, not part of a wider campaign of poisoning, authorities said yesterday.

Westchester County Executive Andrew O'Rourke told reporters in Yonkers, N.Y., that Diane Elsroth, 23, who died Saturday, was killed by someone who placed the poison inside the Tylenol package within the last 10 days. No one has been charged in the death.

Despite reports by police, federal drug officials and the capsule's maker, Johnson & Johnson, that the case is an isolated incident, drug-industry experts said it could force changes in the company's product line.

As Tylenol capsules were being removed from shelves by several national marketing chains, industry analysts predicted Johnson & Johnson will soon begin rethinking Tylenol's position in the marketplace.

The company has been sealing necks and caps of Tylenol bottles and sealing the bottles' boxes since 1982 when seven Tylenol users in the Chicago area were killed by cyanide-tainted capsules.

The most striking fact noted by investigators in Yonkers is that, while the Tylenol used by Elsroth was relatively old and very unlikely to be found in stores, someone must have tampered with the capsules within a week or 10 days before her death.

The batch of 200,000 that included Elsroth's bottle was made in Pennsylvania and shipped by Aug. 22, authorities said, and the poison could not have remained in the capsule for more than about a week without discoloring and eating through the gelatin capsules.

When the company and the Food and Drug Administration sought bottles of that batch from stores for recall and testing, none could be found, even in the A&P store where the suspect bottle is thought to have been bought.

Major grocery and drug chains in Washington, Maryland and northern Virginia said yesterday that none of the products in the batch has been found at individual stores or warehouses. Most area merchants said they think that the incident is isolated and that new safety packaging has eliminated the possibliity of widespread contamination.

With Tylenol by far Johnson & Johnson's most profitable product, the company moved into high gear yesterday to respond to the scare, taking measures to alleviate consumer fears as it had done in 1982.

Within several hours of the announcement of Elsroth's poisoning, Johnson & Johnson set up a consumer hotline for information about Tylenol products and advised Yonkers-area consumers to stop using Tylenol capsules until the investigation is completed.

The company also urged consumers nationwide to stop using Extra-Strength Tylenol in any 24-capsule bottle with lot number ADF916 with an expiration date of May 1987, the batch that included Elsroth's bottle. Those who return Tylenol from that batch will receive credit or an exchange, the company said.

Elsroth was visiting the home of her boyfriend, Michael Notarnicola, Saturday when she said she felt ill, Yonkers Deputy Police Chief Owen McClain said yesterday. The capsules came from a bottle opened by Notarnicola, he said.

The bottle of 24 was found to contain three other cyanide-laced capsules and, after Elsroth's body was found, Notarnicola's mother swallowed a capsule from the same bottle, without ill effect, he said.

An autopsy determined that Elsroth died of cyanide poisoning.

The incident recalled the week of terror in the Chicago area beginning Sept. 29, 1982, when three persons died of cyanide poisoning. Physicians noted the coincidence and called the health department.

Before the episode ended, seven persons from six parts of Chicago died after taking cyanide-laced Tylenol. Each of the victims had recently bought and used a bottle of Tylenol. Three other unpurchased bottles were found to contain lethal doses of cyanide.

Police have not solved the Chicago incidents, but two men described as chief suspects are in prison, according to a spokesman for the Illinois Attorney General.

James E. Lewis was convicted of writing a letter in an attempt to extort money in connection with the killings. Roger Arnold was convicted of killing a local bartender after a fight in which Arnold allegedly accused the bartender of turning him in for the "Tylenol killings," the spokesman said.

No suspects have been found in the Yonkers case, McClain said yesterday, adding that police are questioning several persons, including Notarnicola and his mother.

Financial analysts said Johnson & Johnson's quick reaction to the latest incident was not surprising.

The company's earnings figures for 1985 are to be released today. Financial experts said Johnson & Johnson sold about $530 million worth of Tylenol last year, about 9 percent of the company's total revenues of $6.4 billion. Profits from Tylenol sales are expected to be about $130 million, more than 20 percent of the company's expected profit of $616 million.

Tylenol accounts for about one-third of the $1.6 billion worth of over-the-counter pain relievers sold last year, almost the same market share held by Tylenol before the 1982 scare, which temporarily forced Tylenol sales to drop to 7 percent of over-the-counter analgesics.

The Yonkers incident "will force the company to evaluate its long-term strategy on Tylenol. They will have to try to encourage greater use of 'caplet,' the elongated tablet that is encoded with film to make it easier to swallow. I don't think the company wants to go through these kinds of incidents again," said analyst Gene T. Garguilo of First Boston Corp.

Additionally, Johnson & Johnson could begin producing other types of pain relievers, including ibuprofen, analysts predicted. Tylenol is the company's only pain-relief product