U.S. District Judge Thomas Penfield Jackson said yesterday that D.C. Mayor Marion Barry may have a valid defense to one of three perjury charges against him, saying that a response Barry made to a prosecutor's question during the mayor's January 1989 grand jury appearance was "correct."

Jackson made the remark during lawyers' arguments about final instructions the judge is to give to the jury later this week in Barry's drug and perjury case.

Both sides announced yesterday they had no more testimony to offer, and Jackson excused the jurors until tomorrow, when he has scheduled closing arguments.

The judge is giving both sides today to review more than 5,000 pages of trial transcripts and prepare their arguments. Jackson said he will give each side 2 1/2 hours for arguments tomorrow, and that he probably would give the jury its instructions and allow deliberations to start on Thursday.

The question about the perjury count arose when Assistant U.S. Attorney Judith E. Retchin was talking to the judge about the count alleging that Barry lied when he said he didn't know that Charles Lewis, once a friend of the mayor's and now one of his main accusers, was involved in drugs.

At one point, Barry was asked in the grand jury, "Have you ever talked with anyone else about Mr. Lewis's involvement with cocaine?"

Barry answered, "I'm hesitating because I don't -- I doubt if I did because I don't know of any involvement he had with cocaine."

Jackson said that because the prosecutor's question addressed whether the mayor had spoken to anybody, the defense could say in closing arguments that Barry had not lied to the grand jury.

Jackson indicated yesterday's exchange in court was the first time he had seen Barry's response in that light. "Maybe you shouldn't have raised this issue, Ms. Retchin," Jackson said, adding that he would instruct the jury on the availability of this defense.

Meanwhile yesterday, defense lawyer R. Kenneth Mundy said that before the trial, he may have misunderstood a crucial Jackson ruling limiting his ability to cross-examine government witnesses about the cost and duration of the Barry investigation.

In ruling that Barry is entitled to a jury instruction on the subject of the cost of the investigation, Jackson told Mundy that his pretrial ruling did not completely prohibit the defense from asking about the probe's cost in questioning government agents called by the prosecution.

The judge said his order permitted Mundy "to show that they were biased in that they did not want to lose this particular prosecution." The judge was saying Mundy could have asked the agents whether the probe's cost and duration might have influenced them to try to justify, while on the stand, the government's investment.

Mundy did not raise the issue of the cost of the investigation in cross-examining law enforcement officers who testified for the prosecution.

During the trial, prosecutors fought to keep one of the FBI agents, Ronald Stern, off the stand in the government's case, but they did call agents Peter Wubbenhorst and Frank Steele, plus D.C. police Sgt. James Pawlik. Mundy called Stern as a defense witness, but Jackson's order prevented Mundy from raising the cost issue then.

"I misread the court's ruling, then," Mundy said, "because I did not believe that the door was ever opened to me . . . to go into the costs and duration of this case."

The judge granted a defense request to instruct the jury that even though the government can grant witnesses immunity from prosecution, the defense cannot.

Another issue in the case, whether Jackson would permit Mundy to present a rebuttal to the government's rebuttal, became moot yesterday morning when Mundy announced he would not recall one of Barry's staff members to the stand.

Mundy said on Friday that he would recall staff member Clifton A. Roberson to explain an "absent without leave" form that appears to show he was not in his office on the morning of Sept. 7, 1988. Roberson testified that on that morning, he accepted a job application from prosecution witness Lydia Reid Pearson.

At first, Mundy told Jackson that Roberson would testify, but changed his mind after a morning break. Mundy said initially that Roberson would dispute a handwritten note on the form saying he was out of his office that morning. Mundy also told the judge he would call Roberson's mother, who would testify she and her son attended a gravestone-laying ceremony at her husband's grave at 4:30 p.m. that day. Mundy has maintained that was the time Roberson was out of the office.

"I didn't get a chance to talk to Roberson until this morning," Mundy later told reporters. "I had been told what Mr. Roberson would say, but this morning he was not able to be as precise," he added, about who typed the leave form and jotted down a handwritten note on it saying Roberson was absent from 8:30 to 10:30 a.m. because of the graveside ceremony.

Pearson testified that she delivered a job application and crack to Barry in his office at the Frank D. Reeves Municipal Center that morning.

After Roberson's testimony, prosecutors last Friday produced the form showing the morning absence, and witness Darcell Walker. She was a secretary in the office of mayoral assistant Anita Bonds, where Roberson worked.

Mundy countered with timecards showing Roberson was away from 4:30 to 6:30 p.m. The jury will resolve the discrepancy.

Staff writers Elsa Walsh and Ben Iannotta contributed to this report.