David Trimble has been treated for possible anthrax exposure after a building he visited in the US was found to be contaminated.
The Ulster Unionist leader took medical precautions after being contacted by American authorities claiming the bacteria had been found in the office where he met a congressman.
Mr Trimble had discussions with Congressman Mike Pence at his office on Capitol Hill during a series of meetings with leading politicians.
Stormont culture minister Michael McGimpsey and the UUP's North American representative Anne Smith, also took medication as a precaution, says a party spokesman.