Ten people, including three police officers, were taken to hospital after a chemical attack outside a Leicestershire pub in the early hours of Sunday.
The substance, believed to be ammonia, was sprayed during a huge brawl in which a group armed with baseball bats damaged The Cricketers pub in Aylestone, a suburb of Leicester.
All 10 of the victims were checked over for chemical-related injuries at Leicester Royal Infirmary and later discharged.
Five people were arrested over the incident in Grace Road, during which Leicestershire Fire and Rescue Service was also called out.
Two men aged 53 and 43 were held on suspicion of causing aggravated bodily harm, a 22-year-old man was arrested on suspicion of a public order offence and a 29-year-old woman was arrested on suspicion of criminal damage.
A boy, 15, was also held on suspicion of possessing an offensive weapon.
Inquiries are ongoing. Anyone with information can call police on 101, quoting incident 4 of July 7.