Police in Coleraine searched a premises in Long Commons Coleraine, closing down what is believed to have been a brothel.
The search happened on the evening of Wednesday 26 February. One woman was in the premises at the time of the search but no arrests were made.
In a follow up search in Ballymoney, detectives arrested three women on suspicion of keeping a brothel for prostitution. The arrests were made at a premises on Victoria Street on the evening of Thursday 27 February.
The women, aged 40, 33 and 30 years, were taken from Ballymoney to Coleraine station for questioning. They remain in police custody at present.