Former Green Beret physician Jeffrey Macdonald was confronted Friday with the evidence that federal prosecutors say proves that he murdered his pregnant wife and two young daughters in 1970 at a North Carolina Army base. The jury is expected to begin weighing that evidence Wednesday.

During cross-examination, Assistant U.S. Attorney James Blackburn repeatedly asked Macdonald if he could explain a series of bloodstains, footprints and other physical evidence the government says contradicts his account of the murders. In almost every instance, Macdonald, 35, coolly answered, "No, I have no explanation."

Throughout the trial, the prosecution has dwelt on the location of bloodstains and bits of fibers from a pajama top Macdonald was wearing at the time of the crime. "We are trying the case in the home, where the murders occurred and where the evidence is," Blackburn said. "The defense wants to try the case outside the house."

Blackburn referred to the decision of Macdonald's chief counsel, Bernard L. Segal, to devote relatively little time to direct questioning about the night of the murders. Instead, Segal, who says lawyers have a lot to learn from the theater, created a dramatic setting in which he directed his client to talk about his family life before the murders.

During most of his direct testimony, Macdonald was flanked by an enlarged photograph of his two young daughters in Halloween costumes.

On Thursday, three jurors wept, along with Macdonald, as he read from a letter his 26-year-old wife, Collette, had written to him six months before she was murdered. "Darling Jeff," she wrote, "What a difference a day makes or even a few moments, especially if it takes me from the nadir of despair and returns me to the happy folds of love and light feelings."

Moments before Macdonald read the letter, Segal laid the four murder weapons on a table below the witness stand and read the three count indictment to the defendant. "Did you stab and bludgeon your wife?" Segal asked.

"I never struck Collette," Macdonald responded. He added, "It is not true. . .I never harmed Kimberly [his 5-year-old daughter]."

Segal said he put Macdonald on the stand "to answer the question of what kind of man the defendant was. Because the jury ultimately has to say to itself, "Is the defendant capable of the most absolutely monstrous crime?"

Macdonald is the director of emergency medical services at an inner city hospital in Long Beach, Calif. The murders took place at Fort Bragg, N.C., where Macdonald served as a Special Forces medical officer.

During his testimony, Macdonald said his family was attacked by four intruders who entered his quarters while he slept on the living room couch. "I heard my wife screaming," Macdonald said, as he struggled to hold back tears. "It was just a scream at first. It was Collette's voice. 'Jeff, Jeff, why are they doing this to me.'"

As he tried to get up from the couch, Macdonald said, he met four assailants, one of whom struck him with a club, knocking him back on the couch, Macdonald said he then grappled with the assailants, but was stabbed in the chest and knocked unconscious.

Macdonald said he awoke some time later to discover that his family had been slain. He gasped for air as he told how he roamed from room to room, trying to revive his wife and two daughters. However, he said, their bodies were so riddled with stab wounds that air "bubbled out of their chests."

Shown crime scene pictures of the bodies of his wife and daughters, Macdonald began crying. "All I can remember," he testified, "is a lot of blood."

Sticking soberly to the physical evidence, Blackburn asked Macdonald to explain why, since he had lain stabbed and unconscious in the living room, investigators found little of his blood there. Macdonald gave no explanation.

Neither could explain why his wounds were so minor compared with those of his family.

Both legal teams bring intense feelings to the trial. Brian Murtheh, who was assigned by the Justice Department to assist the 34-year-old Blackburn in the case, has been involved in the prosecution of Macdonald since 1971, when the Army decided to reopen the case after it had cleared Macdonald. Murtheh was then in the judge advocate general's office of the Army.

Murtheh said he had developed protective feelings about Collette and her children over the years. He was present in 1974, when the victims were exhumed to get hair samples from their bodies. "It was a strange experience," the 32-year-old Murtheh recalled. "It was a beautiful day outside. . .It was sort of a spiritual experience, like I was meeting these people."

Segal, 49, has represented Macdonald since the original Army investigation in 1970. He argues that the case is an example of "faceless, mid-level bureaucrats who have no controls on bureaucrats who have no controls on them." Segal said the prosecution has "gone wild" in pursuit of Macdonald.

The government, Segal says, has presented the same evidence it amassed in 1970, when an Army board cleared Macdonald. He regards the new prosecution as zealotry, and has called for a congressional investigation.

"I believe there are rights and wrongs in this world," Segal said Friday. And when those wrongs affect his clients, he added, "I perceive myself as a surrogate. . .I'll strike like an avenging angel."