Health Secretary Patricia Hewitt has been questioned by police over the cash-for-honours inquiry.
Ms Hewitt's interview was described as 'brief'
Ms Hewitt said in a statement that she had been interviewed only as a "witness to their inquiry". Her meeting with detectives lasted less than an hour.
Police are investigating whether party donors received honours in return for cash. All involved deny wrongdoing.
Ms Hewitt confirmed earlier this month that she was among five ministers who had been asked for an interview.
Former Cabinet colleague Alan Milburn has already been questioned, again as a witness.
'Ready to assist'
Downing Street declined to comment on Ms Hewitt's questioning.
Ms Hewitt said: "The police made it very clear to me... I was interviewed as a witness, not as a suspect.
"I've always said I would assist the police if necessary. What I'm really concerned with is the NHS," she told GMTV.
The interview took place in London, though not at the Department of Health, her spokeswoman said.
The honours probe began after it was revealed the Labour Party was given secret loans ahead of the last election, and that some of those lenders had subsequently been nominated for peerages.
The inquiry has been widened to look at the other main parties.
It now appears many, if not all, of the Cabinet members at around the time of the 2005 general election, apart from Tony Blair, have received letters from or been contacted in some other way by detectives.
Police have spoken to 90 people, including some of Mr Blair's closest advisers, and, from the Conservative side, former leader Michael Howard.
Assistant Commissioner John Yates has said he hopes to present a report to the Crown Prosecution Service in January.