Emergency crews responded to a hazardous materials call at a bus terminal in downtown Toronto after a potent cleaning agent was found in a toilet Monday night.

Police received a call for an undetermined yellow liquid which was found in a toilet by a janitor cleaning the bathrooms at 9:51 p.m. The janitor and a security guard were overcome by the fumes from the liquid and called 911.

Preliminary tests have found that the liquid is a cleaning agent, but it's not one that was normally used at the terminal, Toronto Fire district chief Dieter Bertram says.

"It's a little bit of a mystery as to what this yellow fluid is and how it got there," says Bertram.

The two victims were taken to hospital, he added.

Several police cars, fire trucks and other emergency vehicles descended on the terminal located at Elizabeth and Edward Streets. It's often used by Greyhound buses.