Public health officials in the United States have shut down two research laboratories in Atlanta, Georgia after a series of safety breaches.
A health agency report says US government infectious disease labs mishandled dangerous pathogens five times in the last decade.
This year alone, workers mishandled samples of anthrax and the highly-infectious H5N1 avian flu.
In response, the Centers for Disease Control and Prevention (CDC) has closed the two labs involved.
The agency has also temporarily barred high-security labs from transporting dangerous pathogens.
There have been no reported infections from previous cases, and no-one potentially exposed to anthrax has shown signs of illness, CDC officials said.
"These events should never have happened," CDC Director Tom Frieden told reporters on Friday.
"I'm disappointed, and frankly I'm angry about it," he said, adding later he was "astonished that this could have happened here".
The incidents were listed in a report on a potential anthrax exposure in June, which occurred when researchers in a high-level biosecurity laboratory failed to follow proper procedures and did not deactivate the bacteria.
The samples were then moved to a lower-security lab in the agency's Atlanta campus.
"This is not the first time an event of this nature has occurred at CDC, nor the first time it occurred from the [bioterror response] laboratory," the report said.
The CDC only recently learned of a separate incident in May in which a sample of the avian flu was cross-contaminated with a highly pathogenic version of the virus and then shipped to an agriculture department laboratory.
The influenza lab and the bioterror response laboratory have been temporarily closed in response.