WASHINGTON--As the Bush administration approaches a decision on stem cell research, the caricatures have already been drawn. On one side are the human benefactors who wish only a chance to use the remarkable potential of stem cells-- primitive cells that have the potential to develop into any body tissue with the proper tweaking--to cure a myriad of diseases. On the other side stand the Catholic Church and the usual anti-abortion zealots who, because of squeamishness about the fate of a few clumps of cells, will prevent this great boon to humanity. I happen to favor federal support for stem cell research, but unless we treat the opposition arguments with respect, rather than reflexive disdain, we will fail to appreciate the looming dangers--moral and biological--inherent in this unprecedentedly powerful new technique. Embryonic stem cells have been obtained from two sources: tiny 4- to 7-day-old human embryos (blastocysts) from fertility clinics, or specialized tissue from aborted fetuses. The problem with the blastocyst technique is that extracting the stem cells kills the embryo. The embryo is very small, consists of only about 140 cells and has not been implanted in the uterus. But a potential human it is. And it is destroyed. True, it would likely be discarded anyway by the clinic. (Only the most promising embryos are implanted in the infertile mother; the rest are either frozen or destroyed.) Which is why I am sympathetic to the utilitarian argument that one might as well derive some good from the embryo's extinction. Nonetheless, it is important to recognize the gravity of providing government money--and thus communal moral sanction--to the deliberate destruction of a human embryo for the purpose of research. It violates the categorical imperative that human life be treated as an end and not a means. It is a serious objection and should be set aside only with great trepidation. The principal justification for setting aside this objection is practical: Continuing the federal ban on embryonic stem cell research is a losing political proposition. The push from patient advocacy groups, touting stem cells as the answer for millions of the incurably ill, is becoming politically irresistible. With government sanction and government funds, the whole technique would be far more subject to peer review and ethical regulation. The fact is, stem cell research is going on today, but because of the federal ban, it is conducted with corporate money in more shielded and less scrutinized research settings. Scrutiny and regulation are needed because the ultimate societal challenge in stem cell research--largely obscured by the debate over the cells' origin--is the question of the cells' destiny. Stem cells have the remarkable capacity to reproduce themselves indefinitely and thus create millions of replicas. Advocates have tried to stress that they cannot become a backdoor to cloning. Stem cells, they insist, can only produce heart or brain or other tissue (or even organs), but they cannot produce a full human being. It is not at all clear, however, that these cells cannot, under the right conditions, be implanted to produce a full human being. The original 1998 paper by James Thomson announcing the success of his stem cell extraction and propagation technique says these cells have the capacity to produce every type of cell necessary to produce a human organism. Moreover, mouse experiments suggest that adding trophoblastic (placenta-producing) cells from a donor embryo to stem cells could allow uterine implantation and the production of a full human being--and thus a potential army of identical human beings. In theory, one could even manufacture a (BEG ITAL)partial human being, kept artificially alive and harvested for its organs. It gets even worse. In 1998, it was reported that a human nucleus had been implanted in a cow egg cell producing what is called a chimera, a possible hybrid human-cow creature. It was destroyed in its early embryonic stage, but not before giving us a glimpse of horrors that lie within the reach of the new reproductive biotechnology. Stem cell research offers the possibility of a fantastic good: tissue and organs to replace almost any failing part of the human body. It is not the imminent panacea that some of its advocates claim. But in the longer run it will likely produce remarkable cures. It should therefore be allowed to proceed with federal funding and federal regulation--but with extraordinary care and a decent respect for those who, possessed of a keener sense of man's potential for evil and folly, would have us pause before plunging into the biological unknown.