An ethics crisis at one of the world's most successful human embryonic stem cell laboratories has plunged the controversial field of research into a new swirl of uncertainty, with U.S. scientists nervously wondering if the scandal will grow into a new wave of political backlash.

The accusations surrounding Korean cloning expert Woo Suk Hwang of Seoul National University -- the first scientist to grow stem cells inside cloned human embryos -- has already killed a spate of planned studies that sought to prove the cells' medical potential.

But the claims that Hwang may have obtained human eggs for his studies from women who felt pressured to donate are also reigniting a long-smoldering debate in the United States over the ethics of paying young women for their eggs, which are difficult to obtain but essential to the production of stem cells tailored to individuals.

Egg donation, which is generally safe but occasionally leads to serious and even life-threatening complications, has been a wedge issue in the stem cell debates, linking feminists and other liberal thinkers to conservatives who favor tighter limits on stem cell research.

With a wide range of stem cell bills primed for congressional action as early as January, the South Korean meltdown could bolster those seeking stronger limits.

"We're in danger of making women into guinea pigs for this research even before there are any treatments to be tested," said Marcy Darnovsky, associate director of the Center for Genetics and Society in Oakland, Calif., a pro-choice public policy group that favors stronger oversight of egg donation and other biomedical technologies. "We really need clear rules that someone is enforcing."

The imbroglio erupted a week ago when University of Pittsburgh biologist Gerald Schatten abruptly severed ties with Hwang, his collaborator of nearly two years, saying he had evidence that Hwang had obtained human eggs unethically.

Schatten's charges resurrected dormant claims of two years ago, when a young PhD student in Hwang's lab told an interviewer from the scientific journal Nature that she and another young co-worker were among several women who had donated eggs.

At the time, the student's statement alarmed bioethicists in South Korea and abroad. It is a widely accepted principle in medical research that junior members of a research team should not be allowed to be volunteers in studies because such arrangements cannot be truly voluntary.

Concerns about Hwang's experiments were amplified by rumors that the woman had been paid for her eggs, which could have made it even more difficult for a struggling student to say "no."

Hwang quickly denied the story. And before long the student did, too, blaming her poor English for what she said was a misunderstanding. Schatten accepted those denials until Nov. 11, when he said he had evidence that Hwang had been dishonest with him.

Hwang, who in the past two years has become a major celebrity in South Korea and been showered with millions of dollars in government grants, again denied wrongdoing last Monday. But the full explanation that he promised within three days has yet to be released.

This is not the first time Schatten has found himself in the penumbra of an egg scandal. Ten years ago, revelations about criminal practices at a University of California fertility program led investigators to Schatten, who was then at the University of Wisconsin. He had an arrangement to obtain eggs from the clinic in Irvine, Calif., where, it turned out, doctors were impregnating women with embryos made from other women's eggs and distributing excess eggs to researchers without institutional approval.

One Irvine doctor was eventually convicted on federal charges, and two others fled the country to avoid prosecution. Schatten was cleared of any wrongdoing.

Schatten's latest close call arose from his 2004 decision to collaborate with Hwang, who had just succeeded in growing stem cells from cloned human embryos -- a "holy grail" accomplishment that for the first time proved the possibility of growing stem cells genetically matched to any patient.

For Hwang, whose English is marginal, Schatten served as an eloquent translator, spokesman and a link to the centers of scientific power in the Western world. For Schatten, whose own stem cell research had foundered, the deal offered a shortcut to the forefront of one of the hottest fields in biology and into the international media spotlight.

With great fanfare, Hwang and Schatten last month launched an ambitious effort to distribute hundreds of customized stem cell colonies to disease researchers around the world -- including U.S. researchers who have been unable to gain access to such cells under restrictions imposed by President Bush in 2001.

In an interview in his Pittsburgh office last month before the deal collapsed, Schatten's eyes brimmed with tears repeatedly as he talked about the benefits the project might bring to humankind.

The sudden collapse of that endeavor has stunned resource-hungry U.S. researchers, many of whom had been lining up to take advantage of the South Korean's techniques and enviable funding.

George Daley, a researcher at the Harvard Stem Cell Institute and Children's Hospital in Boston, had long-standing plans to visit Hwang in Seoul this week, for example, with the goal of setting up a collaboration.

"We want to do it here, but it's incredibly challenging" given state and federal restrictions on human embryo cloning, said Daley, one of several scientists expressing fears that the South Korean scandal might take a political toll on the field. He was still considering last week whether to cancel that trip in light of Schatten's assertions.

More generally, the evolving situation in South Korea has renewed a long-unresolved debate in this country over the ethics of egg donation for cloning and stem cell research.

With current techniques, it takes dozens of eggs to make a single cloned human embryo, which is destroyed in the process of extracting the stem cells. That means that if the field of therapeutic cloning is to advance -- a field involving the creation of cloned embryos as sources of stem cells that would be genetically matched to particular patients -- a significant number of eggs will be needed both to fuel the initial research and eventually to satisfy the demands of patients.

It is legal in the United States to pay women for their eggs, and in recent years at least two teams of stem cell researchers in Massachusetts have done so to the tune of thousands of dollars per procedure.

Scientists at Advanced Cell Technology of Worcester, Mass., made the decision to pay women only after a long analysis by an ethics board created by the company, said scientific director Robert Lanza. He still thinks it is the right way to go, Lanza said, given the painful injections involved, the uncomfortable egg suction procedure, and the approximately 5 percent chance of a serious case of hormonal over-stimulation, which can require hospitalization.

Others, however, say such payments cannot help but be coercive, especially for poor women who might feel compelled to take on those risks just to make ends meet.

In April, the National Academies, chartered by Congress to advise the nation on matters of science, released a report that recommended against payments for human eggs beyond expenses incurred by the donors, in part because of the "sensitivities" inherent in the creation of embryos destined for destruction.

But the report's impact remains uncertain as research institutions, fertility clinics and the biggest wild card of them all -- Congress -- mull the Academies' findings and the larger issues at hand.

At least six stem cell bills -- including one that would allow broader use of federal funds for the research and another that would allow the creation of cloned human embryos but would ban payment for eggs -- are awaiting action.

Senate Majority Leader Bill Frist (R-Tenn.) has said he intends to get to the bills early next year.

South Korean stem cell pioneer Woo Suk Hwang, center, faces questions about allegations that he pressured women to donate eggs.