The latest man being chased by Chicago investigators in the Tylenol murder case had a grudge against two Chicago stores and has a father who worked in a plant that stocked potassium cyanide, a Chicago law enforcement official said.

Chicago investigators have asked for the FBI's help in locating Kevin J. Masterson, now believed to be in Florida, according to Chicago police.

Masterson has been linked circumstantially to the seven cyanide killings mainly by reports in Chicago newspapers that have come from court records:

* In an affidavit filed before obtaining a search warrant for Masterson's Lombard, Ill., apartment, investigators reported that Masterson said he had a grudge against two stores in the Chicago area, that "many people will suffer," that "Son of Sam will pale by comparison to my action" and that "now is the time to even the score" -- comments he apparently made to a psychiatrist in early September.

* Masterson's father has worked for a chemical company that stocks cyanide.

* A search of Masterson's room failed to turn up any poisons, but did turn up two capsules of Extra-Strength Tylenol.

* The Circuit Court document said the two stores against which Masterson had a grudge were Jewel Food Stores Inc. and Frank's Finer Foods, both of which are chain stores that had more than one bottle of cyanide-loaded Tylenol planted by the Tylenol killer.

A Jewel Food Stores spokesman confirmed by telephone yesterday that Masterson's former wife, Joann, who was an employe of Jewel, filed suit against the store in 1975, saying she had been mistreated by store guards. A settlement of $8,000 was paid four years later.

But, in the affidavit, Masterson is quoted as saying that the settlement was not enough and he was still seeking revenge, according to wire service reports. What grudge, if any, Masterson may have held against Frank's Finer Foods is not known.

Investigators have issued no warrant for Masterson's arrest, saying they want him "only for questioning," and an FBI spokesman said the bureau does not consider him a wanted man because no warrant has been issued.

An FBI official said that nothing links Masterson to the Tylenol killings, and that investigators need more information from Masterson before they can justify an arrest warrant or a judgment that he was not connected to the crime.

Similarly weak and circumstantial evidence had Chicago investigators counting Roger Arnold, 45, as a possible suspect three weeks ago. He told police he had cyanide in his possession until August.

Arnold worked at a grocery store warehouse that distributed products to the stores where poisoned bottles of Tylenol were found. Police found weapons and chemistry books in his home, but officers say they have no evidence linking him to the crimes.

The third major figure sought in the case is a man who called himself James Lewis or Robert Richardson, and who is now being sought on charges of extortion for the writing of a letter demanding $1 million from the maker of Tylenol.

He lived not far from one of the victims, and near two of the stores that sold tainted Tylenol, and he often talked of revenge schemes. But police now dismiss him as a suspect in the killings.