Cloning, or making exact biological copies of any adult human being, may remain forever impossible, four leading biologists told a House health sub-committee yesterday.

But astonishing developments, they said, may still be expected from research on cloning.

"Animal scientists could learn to clone valuable beef or dairy cattle "in a few years," said Dr. Clement L. Market of Yale University, who is working with Colorado State University on just such a project.

Within a short time, said Dr. Beatrice Mintz of the Institute for Cancer Research in Philadelphia, she may be able to produce "at will" large numbers of mice, who - though not clones or identical one to the other - all possess almost any human genetic disease desired.

This would be a huge advance in understanding and learning to treat such diseases, since most of them cannot be produced now in animals.

Drs. Markert, Mintz, Robert Briggs of Indiana University and Robert McKinnell of the University of Minnesota agreed on these and several more points as they were asked to "comment definitively" on the state of the cloning art by subcommittee chairman Paul Rogers (D-Fla.).

Scientists - these scientists said - don't yet know how to produce either human or mammalian clones. No one has produced a shred of evidence, they said that there exists any human clone, and the recent book "In His Image," by David Rorvik that maintains the existance of a human clone is "fiction."

So far, they reported, the only animals that have been successfully cloned are frogs and some other amphibians, like toads and salamanders. Briggs and Dr. Thomas King, now at the National Cancer Institute in Bethesda, produced the first clones - in frogs.

But frog cloning, the scientists agreed, has been accomplished only by starting with cells from a frog embryo. In thousands of attempts, no one has been able to create a clone from a clone of mammals."

In short, they said, there is no known way to create a clone from even a child, let alone an adult. "I am now inclined to believe," said Markert, "that the nuclei of adult cells may have "lost the biological capacity to function" as egg cells, that is, cells from which a new life might spring.

Still, he said, it should be possible to create clones of nonhuman mammals by techniques that would be unacceptable in human beings. He said he has already produced many mouse, rabbit and rat embryos that possess the genes of only one parent. Dr. Peter Hoppe of the Jackson Laboratory at Bar Harbor, Maine, has transplanted such mouse embryos to recipient females, which gave birth to seven liveborn young.

"The next step," Markert said, would be to repeat the procedure with the eggs of these mice, then all their offspring would be genetically identical to one another and their mother - and would therefore constitute a clone of animals.

He and others are already trying to develop the same technique in cattle, Markert said, and "I think it will be possible in other animals, too."

In animals, but not in humans, he added, the breeder could sacrifice all the lines of offspring that don't turn out to be what he needs and continue to clone only those that have valued traits.

The scientists said they could not guarantee that no mad scientist or dictator would ever try this in humans. But they said any legislative attempt to choke off today's research would only curtail future benefits and fail to prevent any misuses.