With the parents' consent these cells could also be used for research, providing many new embryonic stem cell lines for laboratories. The procedure might be even be offered for all embryos generated in fertility clinics when its theoretical risk has been better assessed.

"I can see a day when every fertility clinic embryo has a cell removed and banked for future tissue use or organ replacement," said Ronald M. Green, an ethicist at Dartmouth.

Children born after the preimplantation diagnosis procedure have the same incidence of birth defects as those who did not undergo the procedure. So far, after some 10 years of experience, there is no indication that it causes health problems in humans, said Andrew R. La Barbera, scientific director of the American Society for Reproductive Medicine.

If Dr. Lanza's technique succeeds in generating human embryonic stem cell lines, Dr. La Barbera said, "I suspect that indeed it will become routine to generate stem cells for everyone who undergoes preimplantation genetic diagnosis."

But Kathy Hudson, director of the Genetics and Public Policy Center at Johns Hopkins University, said there was "little data that documents the safety and efficacy" of the preimplantation diagnosis procedure, even after 2,000 births. She urged the American Society for Reproductive Medicine to create a national database to address the safety issue.

The other alternative method reported in Nature today addresses an ethical objection to therapeutic cloning, the idea of treating patients with new tissues generated from their own cells.

The cells would be obtained by taking the nucleus from a patient's skin cell and injecting it into a human egg whose nucleus had been removed. The egg develops into a blastocyst from which embryonic stem cells can be derived in the usual way. Critics say this nuclear transfer technique creates embryos only to destroy them.

To counter this objection, Alexander Meissner and Rudolf Jaenisch of the Whitehead Institute in Cambridge, Mass., have created mouse nuclear transfer embryos that are inherently incapable of implanting in the uterus. They did so by switching off a gene in the donor nucleus that is needed for the implantation process. The gene was switched back on later because it is needed to form the intestinal tissues.

William Hurlbut, a member of the President's Council on Bioethics, has suggested that such unimplantable embryos may satisfy those who say a potential life is being destroyed in the nuclear transfer process. But Mr. Doerflinger, of the bishops conference, told the council last December that this approach did not fulfill his criterion that an embryo should not be created. This is still his position, he says.

Scientists hope that alternative approaches to embryonic stem cell research may ease the political obstacles in their path, but they also wish to avoid being compelled to abandon existing approaches before new ones have been shown to work.

Irving Weissman, a stem cell biologist at Stanford, notes in a commentary in Nature that there have been calls in Congress for a moratorium on generating new stem cell lines until the two new techniques have been adapted to people, a prospect that he describes as "highly speculative."