Plasma Resources UK, which provides blood supplies to the NHS, has been sold to Bain Capital, a private equity firm set up by Republican presidential candidate Mitt Romney.
The move, which has angered some medical experts, is being described as a 'privatisation too far' in some circles.
"It's hard to conceive of a worse outcome for a sale of this particularly sensitive national health asset than a private equity company with none of the safeguards in terms of governance of a publicly quoted company and being answerable to shareholders," said crossbench peer Lord Owen, who helped made the UK self-sufficient in blood supplies when he was Labour health minister in the 1970s.
"Is there no limit to what and how this coalition government will privatise?"
He added: "This is a very delicate question. You get penetration in blood supplies if it's unscrupulously managed. Private equity... what control have you got?"
Even business secretary Vince Cable seemed to express uncertainty about the move this morning.
"I certainly wouldn’t want to see it undermined by any thoughtless commercial activity," he told the Today programme.
"But if the basic principles of the blood donor system are preserved I wouldn’t stand in way of government deriving some revenue from it."
Bain Capital was branded a "job destroyer" during the US presidential campaign, as the media focused relentlessly on how Romney's company shredded jobs in the American heartland.
Romney became very rich through Bain, but left over a decade ago.
The firm has taken an 80% stake in the Hertfordshire-based company with the Department of Health controlling the other 20%.
Bain has paid £90 million and will pay another £110 million in a deferred payment in five years' time.
It has promised to invest £50 million in the service and maintain its headquarters in the UK.
Other competitors included South Korean drugmaker Green Cross Corp and German plasma specialist Biotest.
"Bain Capital was chosen following a fair and open competitive process which looked at who offered the best deal for patients and to ensure future employment at the company," Health minister Dan Poulter said.
Devin O'Reilly, managing director of Bain Capital in London, said: "We are excited about the prospects of PRUK in the growing plasma products industry and are committed to investing in the company to help it reach its full potential in this global market."
Plasma Resources turn plasma in the blood into life-saving treatments for immune deficiencies, neurological diseases and haemophilia.
Plasma is a clear, brown-coloured liquid which acts as a 'storage area' for fluids.
But the plasma is collected from US donors because of concerns of a risk of contamination with variant Creutzfeldt-Jakob disease, the human form of BSE.
The service is completely seperate to blood donation services in the UK, which are unaffected.