The Food and Drug Administration yesterday issued a precautionary warning to consumers against buying "Extra-Strength Tylenol" capsules of batch number MC2880 after the deaths by cyanide poisoning of four persons and possibly a fifth in the Chicago suburbs who had taken Tylenol.

One other person was in "extremely critical" condition in a Chicago-area hospital after swallowing capsules of the medication. Two persons hospitalized yesterday afternoon who hospital officials said had taken "Extra-Strength Tylenol" were listed in stable condition.

Police and the FDA now believe that the poisonings did not result from poisoning an entire manufacturing batch of Tylenol, but rather from the injection of cyanide into individual bottles of the product in the Chicago area. The Cook County medical examiner's office said it was treating the fatalities as "possible homicides."

As a precaution, Johnson & Johnson's McNeil Consumer Products Co., manufacturer of Tylenol, has issued a recall of 93,400 bottles of the red-and-white "Extra-Strength Tylenol" capsules with the lot number MC2880, which is printed in the upper right-hand corner of the capsule bottle's label. The FDA has recommended against buying or using any capsules from this batch.

So far, two bottles of Tylenol containing cyanide have been found, according to the FDA. Both the FDA and the manufacturer have tested samples of the batch MC2880 at the Fort Washington, Pa., plant where it was made and found nothing irregular.

FDA tests of "Extra-Strength Tylenol" capsules in the Chicago area have also proved negative.

One FDA official said that another indication that the source of the cyanide was not the factory was that the batch has been in stores since August without any problem having been reported until now.

Two of the victims, brothers Stanley and Adam Janus, ages 25 and 27, resided in Arlington Heights, Ill. They died within hours of each other Wednesday morning after taking the tainted "Extra-Strength Tylenol" capsules, the Cook County medical examiner's office said.

Theresa Janus, 19, Stanley Janus' wife, also took the pills and is listed in "extremely critical" condition at Northwest Community Hospital.

The first fatality was Mary Kellerman, 12, who had been taking the non-aspirin pain-reliever for a cold. She was found unconscious in her home Wednesday and died after being taken to Alexian Brothers Medical Center in Elk Grove Village.

In nearby Downers Grove, Mary McFarland, 31, of Elmhurst was dead on arrival yesterday afternoon at Good Samaritan Hospital, where she was rushed after taking "Extra-Strength Tylenol." A pathologist's test found cyanide in her blood. The hospital said two suspected victims were in stable condition.

To the southwest in the suburb of Winfield, a spokesman at Central DuPage Hospital said Mary Reiner, 27, who had taken "Extra-Strength Tylenol," died yesterday morning of symptoms resembling cyanide poisoning. Her lab tests were not complete, but the deputy coroner said she had taken two capsules from a bottle containing other capsules laced with cyanide.

The presence of cyanide in the bodies of the other three victims was determined by blood tests, and the Cook County medical examiner's office said that the bottles of "Extra-Strength Tylenol" capsules were tested once it was found that the victims had taken the same pain reliever.

In the bottles collected from the Janus and Kellerman families, some of the normally white, dry powder in the Tylenol capsules had been replaced by a wet, grayish powder containing cyanide, according to the medical examiner's office.

A company spokesman said the recalled lot consisted of 93,400 bottles containing 50 red-and-white capsules each. The expiration date on the batch is April, 1987.

Lawrence G. Foster, a spokesman for Johnson & Johnson, the New Brunswick, N.J., firm that owns McNeil, said he had heard from the medical examiner in Illinois that the capsules "had been pried open and placed back together."