Federal investigators say fake flu shots were given out last week during a health fair at Exxon Mobil's complex in Baytown, Texas.
An Exxon Mobil spokeswoman says a doctor provided the shots and that it was the company's first use of an outside contractor to administer them.
The spokeswoman says the FBI told the company it was "definitely not the flu vaccine."
A prosecutor says it doesn't appear the fake shots were harmful, but that steps are being taken to ensure workers' safety. Exxon Mobil has offered blood tests and counseling to about a thousand employees who took part in the health fair.
The FBI and the Food and Drug Administration are investigating the contents of the syringes and whether others might have received the fake vaccine.