It happened at the old Lipton Tea factory and warehouse in the 500 block of E. Main St. in Streator.
Firefighters responded a little after midnight.
They remained on the scene throughout the day and eventually needed help from around a dozen different departments.
No injuries are being reported, but WMBD 31 is told fire gutted the building and caused extensive damage.
Lipton Tea left Streator in 1964, so the building is believed to have been mostly vacant.
The cause of the fire is under investigation.
Authorities plan to address the situation during a news conference Monday. This is the latest in a string of fires in Streator.
Many of them have been declared arson, but authorities would not say Sunday's fire was intentionally set.