A sick frog was the tip-off to a problem in the Andover Forest neighborhood.
Ken Silvestri, who lives on Brighton Place Drive, said his wife came home Thursday and noticed a frog that hangs around a pond on the golf course behind their home acting strangely.
Then she saw an oily sheen on the water.
The fire department was called and put out booms to stop the spread of the substance, then turned over the job to a private contractor.
Lexington's Division of Water was out early Friday looking for the source of the spill.
Larry Leach, an environmental inspector, said the spill probably originated in the subdivision. He had driven through the area but hadn't found the source.
"There are hundreds of houses and many inlets and manholes," Leach said.
In response to a question from a nearby resident, Leach said the spill was minor and would have no effect on drinking water supplies.
Workers for Purdue Environmental Contracting Co., Inc, the private contractor, continued to put out booms and absorbent pads Friday.
"We'll probably be here most of the day," said worker William Grimes.