As many as 75 people working at the US Centers for Disease Control (CDC) may have been exposed to live anthrax bacteria and are receiving treatment.
The US health agency said researchers in a high-level biosecurity laboratory failed to follow proper procedures and did not inactivate the bacteria.
CDC said the exposure occurred at laboratories in Atlanta at the weekend and no-one has yet shown symptoms.
An internal investigation in under way, the CDC said.
'This should not have happened,' Dr Paul Meechan, director of the environmental health and safety compliance told the Reuters news agency. 'We're taking care of it. We will not let our people be at risk.'
Dr Meechan said it was too early to determine whether the transfer was accidental or intentional.
Symptoms of anthrax exposure include skin ulcers, nausea and vomiting and fever, and can cause death if untreated.
In a statement, the CDC said the anthrax samples were moved from a high-security lab to a lower-security one at their Atlanta headquarters.
'Workers, believing the samples were inactivated, were not wearing adequate personal protective equipment while handling the material,' the agency said.
'The unintentional exposure was discovered June 13 when the original bacterial plates were gathered for disposal and B. anthracis colonies [live bacteria] were found on the plates.'
Although the CDC believes about seven researchers had direct contact with the anthrax it is casting a wide net in who may have been exposed.
About 75 individuals are being offered a course of the antibiotic ciprofloxacin as well as an injection with an anthrax vaccine.
The normal incubation period for anthrax illness can take up to five to seven days, Dr Meechan told Reuters, but there have been cases of illness occurring as much as 60 days after exposure.