It is understood the person suffering from a probable case had just returned from holiday
Northern Ireland's first case of probable swine flu is being investigated, the BBC understands.
Samples have been sent to London after initial tests in Belfast.
It is understood the probable case originated in the County Antrim area in a person who had returned from holiday in a country affected by the virus.
BBC NI's health correspondent Marie-Louise Connolly said cases were "sent to London when there was a likelihood of testing positive".
It could take several days before results are known.
In Northern Ireland, 55 negative tests had been reported before the news of the probable case on Wednesday.
Earlier, Health Minister Michael McGimpsey confirmed two cases, including the probable one, were under investigation.
"So far we have had no confirmed cases of swine flu in Northern Ireland, but that doesn't mean we should become complacent," he said.
"We have been planning for a situation like this for some years and are well prepared. We will continue to make preparations to help us respond in the event of a pandemic."
The minister said leaflets giving information about swine flu were being sent out to all homes in Northern Ireland.
"The Northern Ireland helpline, 0800 0 514 142, is also available for anyone who has been to an affected country like Mexico or the USA and has flu-like symptoms, or for those who have any concerns about swine flu and need more information."