An Altona man was hospitalised with life-threatening injuries after a crash at Laverton on Monday.
Major Collision Investigation Unit detectives are appealing for witnesses after a Kawasaki motorbike and grey Toyota sedan collided in Dohertys Road about 6.20am.
The motorcyclist, 46, suffered severe injuries.
The car driver, a 38-year-old Tarneit man, was taken to hospital with minor injuries.
Police want to speak to any witnesses or anyone with dash-cam footage.
Detectives especially want to speak with a truck driver who was behind the motorcycle just before the collision.
Anyone with information should contact Crime Stoppers on 1800 333 000 or submit a confidential report via www.crimestoppersvic.com.au