The American Red Cross is to ban blood donations from people who have lived anywhere in Western Europe since 1980 as a precaution against mad cow disease.
The organisation hopes to make the restriction apply to all blood collection agencies this week.
The American Red Cross collects about 6.5m units of blood annually - about half of the nation's medical blood supply.
Some experts have disagreed with the ban.
"There is not a lot of evidence that you get this disease from blood transfusions," bioethicist Art Caplan, who heads the nation's top blood advisory panel, said.
At least one collection agency, America's Blood Centers, said the new restriction could be devastating.
"We could lose 25% of New York City's blood supply," said spokeswoman Melissa McMillan.