Three people were taken to hospital following the incident on the A8 outside Larne on Saturday.
Emergency services attended the scene shortly after 3pm, just outside Ballynure village.
A police spokesperson said: "A small blue car and tractor were involved in the crash, just outside Ballynure village.
"Three people were taken to hospital, where they remain at this time receiving treatment for serious injuries.
"One man is in a critical condition.
"The road was closed for some time but has since reopened.
"We would appeal to anyone who was in the area at the time, or who may have dashcam or other footage, to contact us on 101, quoting reference 926 of 21/10/23."