In announcing his policy on embryonic stem cell research Thursday night, President Bush could have merely split the difference between the extremes. Instead, his thoughtful response begins in earnest what is certain to be a long public debate on biomedical ethics. Mr. Bush, who clearly gave the matter much thought and who received much sound counsel, made a decision that is a rational compromise and could ensure that this vital research proceeds with a moral compass.

The decision is to allow federal funding for research on the limited number – some 60 strains – of existing stem cell colonies, or lines, from embryos already discarded by fertility clinics. This plan immediately was greeted with scorn by the science-at-all-costs community and with alarm by some in the right-to-life camp. Moving inward from the edges, one finds most Americans, like Mr. Bush, recognize that responsible scientific inquiry holds the promise of cures to many devastating diseases and that such a promise can be kept ethically if limits are applied on the source of the cells.

It is a plan that was long expected. The administration has been suggesting for months that unlocking federal funds for this research – pegged at $250 million in the coming year – would involve limiting its use to the existing colonies. This suggestion has long been assaulted by the one extreme as a shackle on unfettered research and by the other as allowing mankind to profit from a prior crime against the unborn. One likely outcome of it is to ensure the issue is revisited: As scientists make progress in their research it is nearly inevitable that some will run up against the limitations of the available 60 cell lines, with the promise of dramatic cures on just the other side. The debate then will be based in part on behavior of scientists in the coming months and years.

For now, scientists will have only a relatively narrow array of embryo stem cells available for study, but the science is so new and the benefits so much more potential than reality that the existing strains could offer enough opportunities for advancement. The other side of the argument should recognize that, far from creating a culture of institutionalized callousness toward embryonic life, federal funding rules would bring ethical restrictions to embryo research that now proceeds privately without any. An alternative would be for this research to proceed without any government oversight, for it to migrate abroad.

Further, Mr. Bush demonstrated a keen understanding of the very nature of scientific inquiry. Although he noted that stem cells derived from non-embryonic sources, such as adults and umbilical cords, might in time yield the same results as those produced by embryos, science cannot advance by having promising avenues closed off.

The president’s plan keeps the most promising avenue open as it requires those who follow it to yield to ethics.