A Co Down pensioner was airlifted to hospital after a motorbike crash in Ballywalter on Friday.
The man, aged in his 70s, is being treated in the intensive care unit at the Royal Victoria Hospital, where he remains in a critical condition after the crash on Dunover Road.
Police say they are keen to speak to the driver of a small black vehicle, bearing L plates, believed to have been in the vicinity at the time, who may have witnessed the crash.
The emergency airlift was the first time the newly-launched Air Ambulance has been used in the Ards Peninsula.
The Ballywalter crash was one of three incidents at the weekend which left two men, including the motorcyclist, seriously injured and one dead.
In Lisburn, investigations are continuing after a 71-year-old driver died in a crash in Warren Gardens on Friday morning.
There were no other vehicles involved in the collision.
A post mortem is to be carried out to establish how the man, who has not yet been named, died.
Elsewhere, a man has been released on bail after another weekend road collision, this time in Dunmurry.
It left a man in his 40s in a serious condition in hospital.
An appeal has been launched for witnesses to the crash, which happened in the early hours of Saturday morning at the corner of Glebe Road and Upper Dumurry Lane.
A man in his 60s was driving the car.
He was arrested and questioned by police before being released on bail.