The number of Americans taking anthrax antibiotics at the government's advice is nearing 10,000.
Health officials announced a deal had been made with Bayer Corporation to buy 100 million tablets of Cipro. The government said it will buy the antibiotic for 65p a pill, about half its normal price.
A Centers for Disease Control and Prevention spokeswoman said the number of people the government has prescribed with Cipro or other antibiotics against anthrax is approaching 10,000. Others have obtained medicine from private consultants.
The US postal service is preparing to issue masks and gloves for its 800,000 employees and is testing ways to sterilise the mail.
A fifth location in the US Capitol complex has tested positive for the anthrax bacteria, Capitol police said.
The anthrax was found in an area of the freight elevator of the south-west quadrant of the Senate Hart office building. A letter opened in the office of Senate majority leader Mr Tom Daschle in the building last week was found to be laced with anthrax.
Twenty eight people at the US Congress have been exposed to the disease. A female journalist who worked at the Senate was hospitalised yesterday on suspicion of contracting inhalation anthrax.
Since the anthrax crisis began, two postal workers in Washington and a photo editor in Florida have died.
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