Forensic examiners are testing an unknown substance which forced part of Adelaide's Lyell McEwin Hospital to be shut down early today after a man turned up seeking treatment and staff also became unwell.

Police said the man became ill after he accepted a letter he was told contained a free ticket from a stranger outside the Elizabeth Shopping Centre in northern Adelaide, about 1:00am.

They said he went to the Lyell McEwin Hospital and the medical staff who dealt with him also started to feel ill.

A hazardous materials crew was called in just before 2:00am and the man was put into isolation and part of the health facility cordoned off.

Ambulance services were diverted elsewhere for about three hours as Metropolitan Fire Service officers decontaminated an area.

Police said the hospital had now resumed normal operations and the man had been discharged.

They are seeking more information from the public about the man who handed out the envelope.

They said he was in his late 30s, tall and of slim build, unshaven and with black medium-length hair and wore grey pants and a black jacket.