THE WONDERS of recent biomedical research bring ever more complex ethical challenges; they also call for precise and careful feats of definition. The latest example is research into embryonic stem cells, as-yet-undifferentiated human cells that hold the potential of developing into any one of the body's many tissue types. Scientists working on such cells say they could make it possible to grow cells needed to treat a raft of diseases, from insulin-producing cells for diabetes sufferers and dopamine-producing cells for those with Parkinson's to new tissue for diseased hearts.

But the fact that such cells are derived from human embryos or from aborted fetal tissue has alarmed some members of Congress, who see abortion-related aspects to the research and want to bar the National Institutes of Health from funding it. An amendment barring the NIH from funding any such research could be offered when the institutes' budget, part of the Labor and Health and Human Services appropriations bill, comes to the House floor.

The NIH is currently barred from funding any research in which a human embryo is destroyed. The question raised by such an amendment would be whether public funding should likewise be denied to research on stem cells obtained without government funding. It was in privately funded experiments that stem cells were derived last November, from fertility clinics' leftover embryos that were slated for destruction in any case. The NIH then sought legal advice on whether it could fund experiments done on such cells; the Department of Health and Human Services ruled that it could, as long as the original material was derived by others. (Stem cells reproduce themselves in "lines," so that many experiments can be done over time on cells originating from one embryo.)

There's no gainsaying the discomfort that attends these delvings into the very edges of human development. It was reasonable for Congress not to let public funds go directly to the destruction of human embryos. But when it comes to stem cell research, where the embryos used were being destroyed anyway, and where more and better research might open a bridge to techniques that would not need to use embryos at all--for instance, that might succeed in getting stem cells from ordinary adult cells--the rationale for blocking an explosively promising field falls apart. It then becomes important to weigh the heavy human costs of withholding funding from such experiments--the top-flight academic research forgone, the suffering of uncountable disease victims lengthened. Blocking stem cell research from the scientific mainstream also means losing federal oversight of its ethics and its results.

Stem cells are something altogether new and outside familiar pro-choice and pro-life categories. It makes no sense for Congress to cripple this major area of medical inquiry at its very start.