Jean Harris knew about the other woman in Dr. Herman Tarnower's life, was jealous of her and at one point before Tarnower's slaying had threatened the doctor and his girlfriend, the jury in Harris' murder trial was told today.

In the most damaging testimony so far, Suzanne van der Vreken, Tarower's housekeeper, recounted a visit by Harris to the Westchester County home of Tarnower, inventor of the popular Scarsdale diet:

"Mrs. Harris started to speak about Lynne Tryforos," she said, as Harris, one arm slung over the back of her chair, eyeglasses dangling in one hand, stared icily at her. "She used some words . . . not very nice . . . about that lady . . . she said, 'I will make their life miserable. . . . '"

Then, according to the court reporter's transcript, the Belgian-born Van der Vreken, whose voice is heavily accented, quoted Harris as saying "'I will sue them . . .'"

A number of courtroom observers were left with the impression that Van der Vreken had said "shoot" or "show" -- not "sue" -- and the entire courtroom was confused.

The highly charged and critical testimony was so ambiguous that both lawyers and the judge admitted they were not sure what she had said, and would seek to clarify the testimony Wednesday morning.

"I didn't know exactly what she meant," Van der Vreken continued in her testimony. "I found an excuse and left the room . . . ."

Harris, 57, the former headmistress of the Madeira School in McLean, Va., has been charged with second-degree murder in Tarnower's shooting last March in the bedroom of his $500,000 home in Harrison, N.Y.

The defense has contended that the shooting was a "tragic accident," a suicide attempt gone awry. But the prosecution has maintained that Harris shot Tarnower, her lover for 14 years, in a jealous rage inspired by his relationship with Lynne Tryforos, 20 years Harris' junior.

Testimony today in Westchester County Court by prosecution witness Van der Vreken, who had worked for the doctor for 16 years, substantiated that claim.

Dressed in a boucle knit suit, her hair a mass of gray curls, and a cross prominently displayed outside her blouse, Van der Vreken -- whose eyes often looked angrily with the defendant's -- testified that Tryforos first visited the Tarnower home in 1975, and that, since that time, Tryforos and Harris had been the two women in the doctor's life. She testified that after joining the staff at the Madeira school the headmistress visited the doctor's home less frequently, and Tryforos, who worked in the doctor's Scarsdale Medical Group, came more frequently.

As Harris, chin jutting defiantly upward, continued to stare coolly, Van der Vreken, under direct examination from Assistant District Attorney George Bolen, told the jury that after Harris spent Christmas and New Year's last year with Tarnower in Palm Beach, Tarnower was off on another vacation within a month.

"And do you recall who accompanied Dr. Tarnower on that trip?" asked Bolen.

"Mrs. Tryforos," said Van der Vreken.

Questioning from Bolen also suggested not only that Tryforos had joined Tarnower for that vacation but that she might have seen him immediately after he returned from his New Year's vacation with Harris.

"There came a point when Dr. Tarnower returned home?" asked Bolen.

"Yes," said Van der Vreken.

"Did he fly back?" asked the assistant district attorney.

"Yes."

"Who, if anyone, met him at the airport?" asked Bolen pointedly.

"I don't know," said Van der Vreken, who resumes the stand Wednesday.