The concern focuses on strict new ethics criteria that the National Institutes of Health has proposed. Advocates of stem cell research say that most of the work currently underway passed close ethical scrutiny but that the procedures varied and usually did not match the details specified in the proposed new guidelines.

“It’s not that past practices were shoddy,” said Lawrence S. Goldstein, director of the stem cell program at the University of California at San Diego. “But they don’t necessarily meet every letter of the new guidelines moving forward. We’d have to throw everything out and start all over again.”

Raynard S. Kington, acting NIH director, said that the agency is aware of the concerns but that he could not comment further until officials have reviewed and considered all public comments.

“We know issues like this, among many issues, have been raised, and we will take them into consideration,” Kington said in an interview Friday. The public comment period closes tomorrow, he said, adding that NIH has received more than 20,000 comments addressing almost every aspect of the proposal.