A man has been arrested in connection with the ill-treatment of patients at Muckamore Abbey Hospital.
The 34-year-old was arrested by detectives from the PSNI's Public Protection Branch in the Antrim area on Tuesday morning.
Muckamore Abbey treats patients with learning disabilities and mental health needs.
This is the fifth arrest by detectives investigating the allegations of ill-treatment of patients at Muckamore.
To date one woman and four men have been arrested. Nobody has been charged.
The PSNI is investigating thousands of incidents after allegations were made about abuse of patients.
Detectives have been trawling through thousands of hours of CCTV from the hospital wards.
The footage has revealed an alleged 1,500 crimes on one ward, police said.
More than 30 staff members have been placed on precautionary suspension.
A report commissioned in September 2017, following the reports raised concerns about the safeguarding of adults, provision of meaningful activities for patients and physical health care.
Northern Ireland Secretary Julian Smith has apologised to families over the alleged abuse.