Police Chief Jim Rohr said he gently took possession of the box, which had a date of October 1945 written on it.
"It was a little unnerving," Rohr said. Rohr took it to a nearby field and called the Dane County bomb squad.
The bomb squad took the grenade back to Madison for disposal, the Daily Citizen reported. The mortar round was identified as a 50mm high explosive Japanese "knee mortar."
He urged anyone who runs across similar items to call the police before touching or moving it and officers would come and take a look to determine what should be done.
"We'll be glad to have the bomb squad come and remove it," Rohr said.
Besides a nearly 70-year-old grenade, Fox Lake police also collected 187 pounds of prescription drugs.