A man was subjected to a "brutal and sustained attack" when he tried to help a girl he found lying outside flats in Glasgow's Cranhill area, police said.
The 22-year-old approached the girl at Soutra Place at about 0100 BST on Saturday.
Her male and female friends arrived on the scene and attacked the man, who is now in a serious but stable condition.
Police believe the attackers are local and may have been involved in some other disturbance earlier that night.
Det Cons Andy Hunter said: "It would appear that this chap was returning home when he saw the girl lying on the ground.
"He thought she needed help but it looks like she was drunk and the people she was with decided for no reason to attack the young man.
"He suffered a brutal and sustained attack for absolutely no apparent reason other than trying to going to assist someone he thought needed help."
He said the force was checking CCTV in the area and carrying out door-to-door inquiries.
"When last seen, the group ran into Bellrock Street and also Startpoint Street after the assault," he added.