A 61-year-old hospital worker was on a respirator in "very serious condition" after becoming what is believed to be the first New Yorker to test positive for the dangerous inhalation anthrax, Mayor Rudolph Giuliani said.

More tests were being conducted to determine how the woman, a stockroom employee of the Manhattan Eye, Ear and Throat Hospital, contracted the potentially deadly bacteria, the mayor said.

Health officials were awaiting tests results to make a final determination Tuesday. The cause of the infection was not immediately known.

"We have to assume on the theory and the assumption that it is anthrax," Giuliani said.

New York City has been a focus of the anthrax investigation since an assistant to NBC anchorman Tom Brokaw was infected earlier this month. The city has had four confirmed skin anthrax cases -- all at media outlets -- but none of the more-serious inhaled form.

In Florida, New Jersey and Washington, D.C., three people have died from inhaled anthrax, three others have confirmed cases, and one has survived.

The New York woman, whose identity was not released, started showing possible symptoms of anthrax on Thursday, Giuliani said. By Sunday she was in severe respiratory distress and went to the emergency room of Lenox Hill Hospital.

"There was a rapid progression from Saturday to Sunday," said city Health Commissioner Neal Cohen.

After the initial tests returned positive showing the woman had contracted inhalation anthrax, a hazardous materials unit was dispatched to the woman's workplace for environmental samples. Nasal swabs were taken from 25 workers and those tested were given antibiotics. About 300 full-time employees work at the hospital.

Repeated phone calls to the hospital late Monday went unanswered.

The woman worked near a mail room but didn't ordinarily handle mail, which has been a source of anthrax in Washington, D.C., and New York.

Authorities late Monday were tracing mail routes that lead to the hospital.

City health officials are also contacting patients who visited the hospital over the past two weeks, the incubation period for anthrax.

Earlier Monday, a postal union filed a lawsuit against the U.S. Postal Service to force the closing of New York's biggest mail-sorting center for testing.

"We're simply asking the post office to close the building and make sure it's safe," William Smith, the union president said of the 2-million-square-foot Morgan Processing and Distribution Center. "Test everybody and tell us they haven't been exposed. If that's not done, we shouldn't be in that building."

The Postal Service also announced that absenteeism there had climbed to nearly 30 percent since traces of anthrax were found on sorting machines.

But despite the anthrax difficulties, there have been only "minor, minor disruptions" of mail delivery, a Postal Service executive said.

No postal employees in New York have come down with anthrax.