Two British scientists urged colleagues at the National Institutes of Health yesterday to consider using a new "experimental animal" -- the human placenta -- to test chemicals for toxic side effects.

Strictly speaking, the placenta, which is the baby's life-support system before birth, is not an animal but an organ. It is delivered by the obstetrician minutes after the baby and is routinely discarded.

But the British scientists, Drs. Peter and Rebecca Beaconsfield, argue that in throwing away the placenta, researchers are tossing out a live human organ that is ideal for testing drugs, pollutants and other chemicals. Since it is derived from the same single cell as the baby, its reactions to foreign compounds may predict human side effects better than a rat's would. And, unlike the rat, it is available free.

"We'd like people . . . to understand what an enormous possibility there is in the use of this organ," Dr. Rebecca Beaconsfield said in an interview. She argued that the placenta provides an alternative to present methods of screening drugs and chemicals for side effects: exposing animals or cells kept alive in the laboratory.

For the last year, the Beaconfields have been rushing the placentas of newly born babies to their laboratory, keeping them alive for several hours on a pump that supplies oxygen and studying how their metabolism is affected by doses of aspirin and alcohol. They have found that the placentas' chemical responses to the drugs mimic those of people.

As the baby's source of blood, proteins, hormones and many nutrients, the placenta is more versatile than other organs of the body. Indeed, Rebecca Beaconsfield considers it almost as a scaled-down human being. She points out that it can be viewed genetically as the baby's twin, since both arise from a single fertilized human egg and have identical genes.

"It is an organ that can carry out every physiologic and biochemical function," she said. "I call the placenta a patient. We know its reaction, we know its metabolism, we offer it a foreign compound. If we can see how a foreign compound interferes with [its] normal metabolizing processes . . . we know ahead of time that that's what's going to be a side effect."

American researchers called the idea intriguing, but said it is not yet clear how useful a tool the placenta may be for studying the effects of chemicals on humans.

Dr. Vera Glocklin, associate director for toxicology at the Food and Drug Administration's new drug evaluations branch, was skeptical about whether the placenta could reveal as much about a drug's side effects as tests on animals.

"The principle of toxicology is to dose the whole animal and see which organ system is going to be affected," she said. "Even though it [the placenta] does everything, it may not be representative of the whole body."

Dr. Harry Keiser, clinical director of the National Heart, Lung and Blood Institute, said he thought the placenta could be very useful as a screening device for testing new chemicals, but that it would be up to researchers to discover what kinds of studies it was best suited for.

He said there are serious questions about whether the present time-honored system of testing drugs on several species of animals adequately reveals a new drug's long-term safety in humans. "I really don't think with the current setup, you'd pick up thalidomide," he said, referring to the medication that produced limb deformities in the children of women who took it during pregnancy. "It does nothing to rats or dogs."