Fifteen people were injured, six of them seriously, by a chain-reaction accident involving a city bus, an ambulance and a car on West 125th Street yesterday, authorities said.

An EMS ambulance headed to Randalls Island struck the car at Seventh Avenue at about 11:45 a.m., propelling it into the M100 bus, sources said.

The injured, including the bus driver, were taken to St. Luke’s Hospital.