One issue is whether novel pathogens that perhaps are found in Mexico but not in the United States might enter the plasma supply.

“That’s scary for the end users,” said Corey Dubin, chairman of the Committee of Ten Thousand, which represents hemophiliacs who, like Mr. Dubin himself, got H.I.V. or hepatitis in the 1980s from plasma products, before treatments were introduced to inactivate these viruses.

THE other issue is the safety of donors. Plasma is collected by drawing blood, separating out the plasma by centrifuge, and then returning the red cells, white cells and platelets to the body.

Some Americans have been giving plasma this way as often as twice a week for decades, with no apparent ill effects. But there have not been many studies devised to detect long-term effects. Germany allows people to give plasma this way only 38 times a year, compared with 104 in the United States.

The Mexican donors need visas that require they have a job and a permanent address. The donor’s health is checked on each visit, and each donation is tested for five viruses. Most of the Mexicans interviewed said they had no problems with donating, though it can sometimes leave them lightheaded if they don’t have adequate nutrition.

Reynaldo Bueno Sifuentes, who began giving plasma a year ago when overtime was cut at the auto parts factory where he works, said he did not take the recommended vitamins because they were expensive. He said the donations left him tired, and that when overtime was restored three months ago, he stopped selling plasma.

But many of the Mexican donors are worried less about their health than the possibility that immigration officials will decide the donations constitute work in the United States, in violation of their visas.