Environmental experts were called in when pollution was spotted in a brook.
The Environment Agency and Severn Trent Water sent out teams after a man having lunch at a nearby pub noticed a "milky" discolouration in Whetstone Brook on Sunday afternoon.
The source of the pollutant was traced to a storm drain near the junction of Dog and Gun Lane and Cambridge Road, and a team from Severn Trent Water pumped it clear.
Officials from the Environment Agency have tested the oxygenation levels of the water and have found it to be fine.
An Environment Agency spokesman said: "We don't know what this pollutant was but it has not affected the oxygenation levels, so that is good. We have not got any dead fish on our hands."
Richard Macefield raised the alarm as he enjoyed lunch with his family at the Kaffir Inn along Cambridge Road.
The 46-year-old father-of-one, from Evington, said: "The children crossed a small bridge over the brook at the back of the pub to get to a playground.
"I walked over and saw the water was a milky colour. It looked like someone had poured millions of gallons of milk into the brook. I have no idea what it was but it did not look good, so I called the water company."
A spokeswoman for Severn Trent Water said the storm drains were not connected to the domestic sewage system. She said: "The Environment Agency will carry out an analysis to establish just what it was which affected a small stretch of the brook.
"We are assisting them with inquiries with local businesses to try to find out where it came from and how it got into our system.
"Our aim is to try to prevent a repeat of this pollution."