New tests on five Food and Drug Administration mailrooms, an office linked to Health and Human Services, and the clothing of a Bronx woman who died of anthrax have turned up negative for the bacteria, overturning earlier tests that were "presumptively positive,” officials said yesterday.

"Everything is negative,” said FDA spokesman Lawrence Bachorik, who also suggested that employees of his agency who were given antibiotics based on the early tests stop taking the medicine.

The announcement, confirmed by the national Centers for Disease Control and Prevention in a news conference yesterday afternoon, opens new questions about decisions being made in determining which offices should be closed and cleaned, which shouldn't and who should be on antibiotics.

Initial testing, which can be done in a matter of hours, involves microscopic scrutiny of material taken from various areas within a building, as well as the use of a "rough” test that detects the presence of anthrax proteins, according to the CDC's lead anthrax investigator, Bradley Perkins.

But the test is not precise, and the only basis for finally declaring a site contaminated is cell cultures. Swabs from suspicious surfaces are placed in petri dishes where laboratory scientists try to grow anthrax bacteria. If anthrax grows, contamination is confirmed.

However, cell culture testing takes 24 to 48 hours, and rapid assay findings often reach the ears of the media and frightened building inhabitants long before results of the more precise tests become available. One outcome of that may be that some Americans are being placed on antibiotics needlessly, some experts said.

For instance, Dr. Michael Osterholm, a bioterrorism expert and epidemiologist at the University of Minnesota, argued that anthrax spores are unlikely to remain in a well-vented space for more than "a few hours,” even if early tests do suggest they're there.

The exceptions, he said, are places where high-powered machinery is at work, such as mail-sorting devices. Otherwise, Osterholm said, "I do not understand why buildings are being closed in the Capitol. I think it sends a very bad message to the public.”

Yesterday's negative results include four FDA mailrooms in and around Rockville, Md.; another mailroom at Federal Office Building 8 in Washington, D.C., which houses some FDA offices; and at the Cohen Building in Washington, where offices for Health and Human Services and the Voice of America are located.

The CDC and the New York City Health Department also said yesterday that new tests showed that hospital worker Kathy Nguyen's clothing did not have anthrax spores on them, as reported earlier.

While Perkins said the CDC is "trying to find the right balance point,” he agreed that most findings of anthrax in individual buildings offer little risk to workers.

"We think that a variety of places will have environmental contamination, and we believe that contamination should be cleaned up,” Perkins said yesterday. "But we do not believe that most environmental contamination represents a substantial risk to human health, particularly for inhalational disease.”

Meanwhile yesterday, federal officials said anthrax was detected inside the Pentagon and promptly removed, while the cleaning of the Senate office building where an anthrax-packed letter was opened proved more complicated.

The Hart Senate Office Building, which was shut down after a letter contaminated with anthrax was opened there on Oct. 15, was to remain closed until cleanup by the Environmental Protection Agency. The agency said that cleanup was to be accomplished by pumping chlorine dioxide gas into the building, but the effort was being delayed while the method was further evaluated, according to spokeswoman Tina Kreisher.

Kreisher said a peer review group got together at EPA over the weekend, and "they have reservations about making sure that we're doing this properly, and we're going slowly to make sure we're doing this right.”

In the meantime, she said, the EPA will use the liquid form of chlorine dioxide or bleach or Sandia foam on some surfaces, including stairwells and elevators in the building.

"We're not ruling out the gas,” she said. She said the gas method was being tested at a sealed trailer near the Brentwood Post Office facility in Washington, where anthrax also was found.