Around 84 scientists may have been accidentally exposed to anthrax at a US government health facility in Atlanta, Georgia.
Anthrax is a disease caused by a germ that lives in soil, and gained notoriety after a spate of US mail attacks in 2001 killed five people.
The scientists have been given antibiotics and are being monitored for signs of illness, the Centers for Disease Control and Prevention (CDC) said in a statement.
The potential exposure occurred at a high-security lab "after established safety practices were not followed", said the CDC.
Samples of Bacillus anthracis (anthrax) were being prepared for research at lower-security labs "to yield new means of detecting dangerous pathogens in environmental samples," it said.
"However, the lab used a procedure that did not adequately inactivate the samples."
The CDC said the staff members at risk were being monitored out of an abundance of caution, though the risk of infection was "very low".
"Based on the investigation to date, CDC believes that other CDC staff, family members, and the general public are not at risk of exposure and do not need to take any protective action," it said.