WASHINGTON -- In unusually emphatic and optimistic terms, scientists told a Senate hearing this week that research with human embryonic cells was likely to yield important medical benefits and should be supported by the federal government.

"It is not too unrealistic to say that this research has the potential to revolutionize the practice of medicine and improve the quality and length of life," said Dr. Harold Varmus, director of the National Institutes of Health.At the hearing, the scientists who announced last month that they had isolated and cultured the cells explained how they could be grown into any desired tissue of the body and used to treat almost any disease, particularly those like Parkinson's disease and juvenile diabetes that are caused by deficiency of a specific type of cell.

"There is almost no realm of medicine that might not be touched by this innovation," Varmus said.

But use of the human embryonic stem cells also raises ethical issues. The cells are widely regarded as meriting a special moral status. They are close cousins to the fertilized egg, although they cannot by themselves develop into an infant.

While some ethicists told the panel that the practical benefits of treatments using the cells should outweigh moral qualms about their use, Richard Doerflinger, of the National Conference of Catholic Bishops, said that "scientific progress must not come at the expense of human dignity." He also warned against use of the cells unless they had been derived from spontaneously aborted fetuses.

The purpose of the hearing was to explore the implications of the new research in light of a ban on using federal funds for embryo research.

The human embryonic stem cells described last month were developed by academic scientists who were supported by Geron Corp. and received no federal funding. University scientists, who receive most of their support from the NIH, would like to work with the cells but will not be able to do so unless the ban on the financing of embryo research is lifted.

Evidently hoping that the ban will not be introduced for a fourth year, Varmus and other scientific witnesses were unusually eloquent in describing the expected benefits of embryonic stem cell research.

The cells are the all-purpose genetic material from which all of the body's tissues are molded. Now that cells can be cultured in the laboratory, scientists hope to channel them into any desired type, replacing or relieving the need for transplantable donor organs and providing novel treatments for many kinds of disease.