That followed news last week that anthrax had been found in a remote mail screening area for the court in the Maryland suburbs.
Court officials said they received Sunday night the results of a Friday night sweep of the building.
"The results were negative except for one area in our mailroom on the basement level of the building," the court said in a statement. "All tests in the rest of the building were negative."
The statement said additional testing was being done Monday based on the positive finding in the mailroom.
Meanwhile, the Supreme Court heard argument away from its usual home on Capitol Hill Monday, the first time since the British burned Washington in 1814 that the justices have been driven from their courtroom by a threat.
At least some of the justices have been started on an antibiotic regimen as a precaution, though court officials refuse to confirm.
Scores of court personnel have been tested for anthrax exposure and are awaiting test results.
Monday morning, the nine justices heard argument in the ceremonial courtroom at U.S. District Court, down Constitution Avenue from Capitol Hill.
It was the equivalent of the president operating from a mayor's office, or Congress meeting in a state legislature because of the anthrax threat.
Court Marshal Pamela Talkin issued her usual warning in the unusual setting: "Oyez, oyez, oyez. All persons having business with the Supreme Court of the United States are admonished to draw near and give attention, for the Supreme Court is now sitting. God save the United States and this honorable court."
Looking more tentative than usual, and stepping more carefully, the justices took their usual seats in relation to each other, with Chief Justice William Rehnquist taking the center chair behind the bench in the strange courtroom and the other justices sitting on each side of him according to seniority.
In his opening statements, Rehnquist made no mention of the anthrax threat, but remarked that it was the first time argument had been heard away from the Supreme Court's present home "since the building opened in 1935."
Rehnquist thanked the personnel and judges of the federal courthouse for making room for the Supreme Court. He added that the high court would meet in the same place for argument Tuesday.
When the classical building on Capitol Hill was completed at a cost of $15 million in 1935, it was the first time the Supreme Court had a building all to itself.
In 1814, after the invading British burned the Capitol, Congress set up cramped quarters in the Blodgett Hotel.
Since at the time the Supreme Court had no home of its own -- only a courtroom deep in the bowels of the Capitol -- it also had to move. The justices held court "in several places" in Washington, according to the court archives.
The court moved back into the Capitol when its burned-out shell was rebuilt, and stayed until 1935.
Last week, the Supreme Court building was one of the few open to the public on Capitol Hill because of the threat of anthrax.
That threat held down the number of tourists visiting the court to several hundred people, none of whom came near the Supreme Court mailroom where mail had been delivered from the remote site.
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