A motorcyclist was seriously injured after colliding with a JCB tractor, police said.
Emergency services were called to the junction of London Road and Shelford Road, near Harston which is close to Cambridge, at about 17:00 BST on Thursday.
Police said the blue Honda NC750X motorbike was involved in the collision with the tractor which was heading in the opposite direction.
The force said the rider, a 44-year-old man from Cambridge, suffered life-threatening injuries and was taken to Addenbrooke's Hospital.
The driver of the JCB, a 29-year-old man from Newton, was uninjured and remained at the scene after the crash.
Sgt Dan Newton said: "I am appealing for anyone who may have witnessed the collision or has dashcam footage to get in touch with us."
