A 22-year-old man had part of his right ear bitten off in an incident in Northampton.
He was taken to Northampton General Hospital and then to Stoke Mandeville Hospital in Buckinghamshire for treatment.
The attack took place in George Row on Saturday.
Four men were later arrested: a 30-year-old man from Wellingborough man and three men from Rothwell aged 28, 31 and 44.
They were later released on bail pending further inquiries.
Police are appealing for witnesses to the incident or anyone with any information to call Northampton Police on 01604 700700.