Scientists have transferred the rat growth-hormone gene into mice to make giant "super-mice," a feat that someday might lead to production of "super-animals" of many kinds, biologists announced yesterday.

The mice -- nearly twice the size and weight of their normal brothers and sisters -- "point the way to a new era in genetic engineering," according to an announcement from the Salk Institute for Biological Studies, which produced the specimens in cooperation with scientists at the University of Pennsylvania, University of Washington and University of California at San Diego.

The first of the new mice were born in Philadelphia last May at the University of Pennsylvania School of Veterinary Medicine.

Super-animals, like super-cows or super-pigs, might produce more meat and milk to feed the world's growing population, the scientists speculated in a paper being published today in the important scientific journal Nature.

Animals with transplanted genes might also be used as biological "factories," they said, to make hormones or other natural products useful to humans.

Even more important, the scientists said, is the basic demonstration that a gene carrying a useful trait can be united with another mammal's gene, with the union producing the new trait. This could lead to future experimentation in combining genes of many kinds, including human genes.

Such gene transfer, said the scientists, could also become a powerful way to study the method by which all genes are regulated and "expressed," or turned on. Understanding this is one of modern biology's main goals.

Genes have been transferred in the past from one animal to another. But in none of the previous experiments, these scientists said, was there any indication that a gene transferred from one species to another worked in a "meaningful" way.

A gene, human or animal, is a long double chain of chemical molecules that together make a strand of DNA, or deoxyribonucleic acid.

The collaborating biologists -- Richard Palmiter, Ralph Brinster, Robert Hammer, Myrna Trumbauer, Michael Rosenfeld, Neal Birnberg and Ronald Evans -- first cloned or grew the rat growth-hormone gene in the laboratory, then deleted one segment by chemical methods.

They then substituted a crucial section of mouse-gene DNA for this segment, a section that acts as a "promoter," or go-signal, for genes.

These modified rat genes were injected into 170 fertilized mouse eggs that were then implanted into seven foster-mother mice. One mother failed to give birth, but seven of 21 mice born last May to the other six mothers carried the new rat-mouse gene. Six of these grew significantly larger--up to 1.8 times larger -- than littermates that did not carry the new gene.

Two of the large male mice were mated and fathered litters of large mice, proving that the new combined growth-hormone gene can be passed on. One of these mice fathered 19 offspring, 10 of which are also super-mice. The other male produced fewer large offspring.

What science is doing, said Brinster, is opening a new field, and "I think our efficiency will get greater as time passes."

Scientists said it is highly unlikely that anything like the same kind of growth-hormone transfer would ever be repeated in humans. A human growth-hormone gene might be placed in some future super-animal, however, to produce a plentiful supply of human growth hormone to treat stunted children.