OLD SPRING HARBOR, N.Y. — At a conference on stem cells held here in March, researchers reported a lopsided harvest of results.

Adult stem cell research is racing ahead, but study of human embryonic stem cells is lagging because a major player, the host of academic researchers supported by the National Institutes of Health, has been sidelined. Under guidelines drawn up in the Clinton administration, the N.I.H. has accepted the first round of applications for research on the cells, but the Bush administration is reviewing the legal basis of the research and may act to halt federally financed researchers from using the cells.

Should it do so, the first fruits of human embryonic stem cell research are likely to be reaped abroad in countries like Britain, Israel and Australia, and to a lesser extent by American researchers who do not rely on N.I.H. money.

Stem cells are unspecialized cells that can spin off the mature cells of the body's tissues while maintaining their own numbers. Biologists believe that both embryonic and adult stem cells hold enormous promise for repairing the body's tissues. On March 30, two teams of researchers reported using bone marrow stem cells to repair the hearts of rodents in which heart attack damage had been induced.

But researchers do not yet know which kind of cell will prove more powerful in regenerating the body's tissues, and want to try both. It may well be that embryonic stem cells will prove better for some kinds of therapy, adult stem cells for others.

Opponents of abortion object to the use of embryonic stem cells because they are obtained by destroying an embryo. Instead, these critics argue, adult stem cells should be used exclusively, a position that is apparently shared by President Bush. "I believe there's some wonderful opportunities for adult stem cell research," he told reporters on Jan. 26. "I believe we can find stem cells from fetuses that died a natural death. But I do not support research from aborted fetuses."

Many biologists regard it as premature to close off study of embryonic stem cells, saying there is no way of knowing whether adult stem cells will prove better until the two have been compared. Last month 112 college and university presidents wrote to the secretary of health and human services, Tommy G. Thompson, saying that discovery of the cells was "one of the most promising biomedical developments in years" and held "exceptional promise" for treating intractable diseases.

Mr. Thompson, while governor of Wisconsin, praised the work of Dr. James Thomson of the University of Wisconsin, one of the two biologists who reported in November 1998 that they had derived human embryonic cells. The other was Dr. John Gearhart of Johns Hopkins University. His department is now reviewing the legal opinion it issued during the previous administration that said, in effect, that federally supported researchers could use but not derive the cells. The ruling was criticized by abortion opponents in Congress as an attempt to sidestep the Dickey amendment, a rider attached annually to appropriations bills that bars federal financing for any research in which an embryo has been destroyed.

The N.I.H. is reviewing its first set of applications for human embryonic stem cell research and will approve research under special guidelines it drew up last year, unless the administration rules otherwise.

Adult stem cells are found in several different tissues of the body, including the bone marrow, skin and brain. Contrary to previous belief that each could generate only one kind of tissue, the cells now appear remarkably versatile, as demonstrated in the use of blood-forming stem cells from the bone marrow to generate new heart tissue in mice.

Embryonic stem cells come in two types. The best known are those derived by Dr. Thomson from surplus embryos generated in fertility clinics but not needed by the prospective parents. Large numbers of these futureless embryos are stored in freezers throughout the country. The embryo, known at this stage as a blastocyst, is a microscopic hollow sphere holding a mass of about 200 cells. These cells, when cultured in the laboratory, are known as embryonic stem cells. They are thought to be capable of forming all the tissues of the body when exposed to the appropriate signals.

The number of blastocysts that will be used if research goes ahead is likely to be strictly limited. A remarkable feature of the cells is that they can grow and multiply indefinitely. Dr. Thomson said that in principle the cells from a single blastocyst could supply all researchers' needs, although in practice he thought about 10 lines would be needed to make sure that typical cells were being used. Researchers generally prefer to use a standard line of cells so that their results can be compared with others'.

Some biologists have suggested it might be necessary to establish a large bank of different embryonic stem cell lines, so as to be able to find an immunological match for any patient. But even adult stem cells seem to present little provocation to the immune system. Dr. Thomson said he believed a tissue bank of embryonic stem cells would be unnecessary because immune tolerance could be achieved by getting the cells to take up residence in a patient's bone marrow, then transplanting them wherever needed.

When a fetus starts to develop from embryonic cells, a pocket of the cells is set aside so they can later generate the eggs or sperm. Embryonic germ cells, as they are known, share many of the properties of embryonic stem cells, including the ability to differentiate into the different tissues of the body. Dr. Gearhart has isolated embryonic germ cells from fetuses that were aborted for the pregnant woman's health. Although President Bush seemed to be referring to these cells, not the ones derived from blastocysts, the germ cells may be regarded as less problematic because the fetus was aborted to protect the woman, not for the sake of generating the cell line.

Dr. Gearhart believes that if the legal opinion underpinning the N.I.H. guidelines is reversed, "Our work goes back to being eligible for funding anyway." It would be ineligible, in his view, only if the administration specifically prohibited it.

Meanwhile in Britain, where much of the early study of embryonic stem cells was done by researchers working on mice, Parliament this January adopted recommendations that allowed biologists both to derive and to study human embryonic stem cells. Dr. Martin Pera, of Monash University in Australia, said at the Cold Spring Harbor conference that he had established four lines of human embryonic stem cells, which he would distribute under a standard agreement. The cells were extracted from blastocysts by colleagues in Singapore and derived in compliance with N.I.H. guidelines.

A spokesperson for WiCell, a nonprofit entity set up to distribute Dr. Thomson's cells, said that about 30 institutions in the United States and abroad had received the cells.

The first forays into financing embryonic stem cell research have been taken by the Howard Hughes Medical Institute, which is supporting one researcher, and the Juvenile Diabetes Foundation, which has financed three. Dr. Thomas R. Cech, president of the Hughes Institute, said a decision by the Bush administration to block federally financed research "would be a most unfortunate circumstance because it would mean the people doing it openly would have to discontinue their work, whereas the people doing it secretly with no oversight and no public discussion would be able to continue, so I would find that to be ironic."