The magazine Nature reported in May 2004 that a member of the team to first successfully produce a human embryonic stem-cell line from cloned human cells was also a donor of eggs used for the research.

The magazine, a leading scientific journal, said its report was based on an interview with the woman, and that the incident raised questions about whether she profited professionally from being a donor. Bioethicists were quoted as saying that there should be an arms-length relationship between researchers and donors.

The report was denied by the team's leader Woo-Suk Hwang.

But Schatten said he received information very recently that questioned the team's denials of the Nature report.

"Regrettably, yesterday information came to my attention suggesting that misrepresentations might have occurred relating to those oocyte donations," Schatten said in a statement dated Nov. 12. "Compliance concerns with ethical practices for obtaining donated oocytes in their 2004 report, and the resultant breach of trust, are the issues that force me to make this decision."

An oocyte is an egg that has not yet undergone maturation.

But Schatten said his decision did not undermine the importance of the research, which is sharing stem-cell lines from patients with spinal cord injury, juvenile diabetes and genetic immunodeficiency. Hwang's group in Seoul is the only place in the world where those lines have ever been made.

"I continue to believe the scientific accomplishments of Prof. Hwang and his team at Seoul National University, including those in which I had been involved, are landmark discoveries accelerating biomedical research," the statement said.