A 21-year-old man has received treatment at Kettering General Hospital after being hit with a broken glass bottle following a dispute at a bar in the town.
The victim was treated for a cut to the head which he sustained in the assault at Decades bar in Carrington Street.
He had intervened in an argument between a man and a woman, prompting the man to hit him.
The incident took place between 2am and 2.30am on Saturday, April 5.
The offender is described as black, 5ft 5in, clean shaven with a skinny build and tightly cropped hair. He was wearing a black jumper and blue jeans.
Officers are questioning a man in connection with the incident but have appealed for any witnesses to come forward.
Witnesses, or anyone with information, should call Northamptonshire Police on 101. Alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.