It happened on Broadstone Way in Holmewood, Bradford at abut 4.10pm on Thursday, June 3.
A car, believed to be a silver Ford Focus, had collided with a quad bike.
A passenger on the quad bike, an 18-year-old man, suffered serious head injuries.
He was was pronounced dead at the scene.
The 19-year-old driver was taken to hospital.
He remains in a serious condition.
Two men, aged 19 and 30, have been arrested on suspicion of murder.
They remain in police custody.
Detective Chief Inspector Tony Nicholson of the Homicide and Major Enquiry Team, who is leading the investigation, said: “We have launched a murder investigation regarding what has clearly been a very serious fatal collision in Bradford and are carrying out a number of active enquiries today.
“Two arrests have been made and there are further people we are seeking to identify and speak with regarding potential involvement in this matter.
“I am appealing for anyone who witnessed the collision or that may have dash cam footage, in particular of the silver Ford Focus to contact the police,
“Anyone with any information that could assist the investigation is asked to contact Homicide and Major Enquiry Team via 101 quoting crime reference 13210276071 or call the independent charity Crimestoppers anonymously on 0800 555 111."