FIRE fighters and Environment Agency officers rushed to the scene after a brook at Kempsey, near Worcester, became contaminated with a bright green substance.
But the pollution turned out to be a harmless - though eye-catching - dye.
The pollution in Hatfield Brook was discovered by resident David Harrison, who is also the village's district councillor, during a walk earlier this evening.
He said: "The substance is bright lime green. I called it straight in and the fire brigade and Environment Agency were on the scene very quickly. They're now putting in a boom to prevent it getting into the River Severn."
A fire crew from Worcester and the service's environmental protection unit, which is based at Stourport, have been sent to the scene.
UPDATE: Phil Major, hazardous materials advisor at Hereford and Worcester Fire and Rescue Service, told the Worcester News at 8.20pm that the substance has been tested and found to be a harmless dye used as a drain tracer.
He said: "It looks very alarming, so it's no surprise that people called us, but it is completely harmless to the environment.
"Once we confirmed that it was harmless, we left the matter in the capable hands of the Environment Agency and went on our way."
Mr Harrison described all concerned as doing an "excellent job".
The initial call came at 6.40pm, and the whole operation was finished by 8.15pm.