AMARILLO, Texas (AP) _ Attorney Richard ''Racehorse'' Haynes, who won acquittal in oilman Cullen Davis' murder trial here 11 years ago, gave opening arguments Monday for a woman accused of plotting to kill her boss' wife.

Davis, a Fort Worth millionaire, was accused of killing his stepdaughter, Andrea Wilborn. This time, Haynes represents Jane Ada Calloway in the murder of Judy Saragusa, 39, who was shot to death at home in Houston on Sept. 2, 1987.

Ms. Calloway, a twice-divorced, 36-year-old and the former executive secretary of businessman Michael Saragusa, is accused of hiring Leon Hawkins Jr. to kill Mrs. Saragusa.

During pretrial hearings, Saragusa admitted he had had an affair with Ms. Callaway, but said it had ended two months before his wife's slaying in the bedroom of the couple's mansion.

Ms. Calloway is free on $50,000 bail and had to be ordered by a judge to stop telephoning her ex-boss.

After police identified a car seen leaving the murder scene as belonging to Ms. Calloway, she said she lent her car to Michael Richardson, the alleged middleman in the murder plot, on the night of the shooting.

Richardson has agreed to testify for the state under a plea bargain in exchange for a 10-year prison term, said his attorney, John Ackerman.

Richardson's stepbrother, Leon Hawkins Jr., surrendered to police two days after the shooting and was charged with capital murder.

Haynes said the trial ''may develop a lot more questions than are answered,'' but said there are others who had motives to kill Mrs. Saragusa.

Haynes said Ms. Calloway was used by her boss to handle his mounting personal bills and ''run them through the company,'' Quality Beverage Co. of Houston.

Following the killing, Haynes said, her loyalty to Saragusa led her to protect him by lying to police about her love affair with him and about the car seen leaving the murder scene, Haynes said.

Saragusa began the affair with Ms. Calloway and was involved in an unhappy marriage, Haynes said, and Mrs. Saragusa was ''domineering and abrasive and created discussion in Quality Beverage about her sexual practices which annoyed Mr. Michael Saragusa.''

His wife, who was contributing to Saragusa's $4 million to $5 million debt, spent most evenings with her horses and a female friend, Haynes said.

''Judy spent thousands and thousands'' on her horses and remodeling the couple's $1 million Houston home, he said.

Ms. Calloway was portrayed in opening arguments as a capable secretary who craved wealth and status and a ''perfect sitting duck'' who was pursued and seduced by her unhappily married employer, to whom she had great loyalty.

Prosecutor Chuck Rosenthal said Ms. Calloway wanted Mrs. Saragusa dead ''because she was covetous of her possessions and covetous of her social position in Houston.''

She wanted the wealth and status associated with being the wife of a wealthy co-owner of the lucrative wholeale liquor business, he said.

Saragusa's finances were expected to be a principal component of both defense and prosecution testimony, with several Houston bankers on the witness list.

Rosenthal charged that Ms. Calloway, who had the authority to transfer money among Saragusa's accounts, transferred $135,000 the month before the slaying to an account she controlled and used some of the money to pay Hawkins.

Haynes said that, despite a $400,000 annual salary, Saragusa faced mounting debts and was sometimes overdrawn several thousand dollars at the bank.

Testimony is expected to last more than a month. State District Judge Michael McSpadden ruled that a gag order, issued before the trial for attorneys and witnesses, continue during court proceedings.

The murder case was moved here from Houston on a change of venue, the same circumstance that brought Haynes from Fort Worth to defend Davis in 1977.