Last June and just a few weeks ago, men dying of a strain of hepatitis that would most likely have destroyed a human organ received livers from baboons. Both times, the activists sprang into action.

I am an animal-rights supporter. I donate money to groups opposing some forms of medical experimentation on animals. I have argued for years, and will today, that animals are sometimes mistreated in such experiments.

But in my chest there beats a heart that used to belong to another person. Now it is mine, and it has already given me nearly four years of a complete and satisfying life after I fell victim to a deadly disease that destroyed my heart.

Had there been no experimentation on dogs, sheep and pigs, you would now be reading another essay by another person. I would have died in 1989. I almost did die at Temple University Hospital in Philadelphia until-just days before my shapeless, bloated heart would offer its last convulsive beat-a donor was found. I awakened from the operation with a strong, 27-year-old heart and a new life at the age of 49. Today, I am 53, healthy and happy. My ultimate life span is unknown. But so is everyone else’s.

My doctors had to wait until almost the last moment before a suitable donor was found. But now, animal-to-human “xenografts” are nearing routine use as bridges to transplantations and promise a way to destroy the biggest killer in transplantation medicine-the deadly shortage of donor organs.

Americans refuse to donate enough organs to help the rest of us to stay alive. The number of heart transplants has peaked at about 2,200 a year. Thousands more of us could be saved from an untimely death. (About a third of recipients are under 44.) But the donor supply has plateaued. The reasons for this mystify transplantation experts. Twenty to 30 percent of those who need a heart transplant will die while they are waiting.

Biomedical engineers are working hard on mechanical replacements for hearts, but the human body does not take kindly to such machines. They present problems that may never be solved, and no mechanical solutions at all loom for bad livers, kidneys and lungs.

Anti-rejection drugs, however, can successfully allow animal organs to be used in humans. And there is hope that animal hearts, livers and perhaps even lungs could someday permanently replace diseased organs. In the future, genetically altered animals may be bred to provide matched organs for dying humans.

But this may never happen if animal-rights activists, with whom I agree on many things, convince society that it is wrong to sacrifice a pig or a baboon or a monkey to save a human. There are already numerous precedents for the use of animals to save lives. Insulin, which keeps diabetics alive, came from animal organs. Many Americans, including my mother, had their lives extended for years through the use of pig heart valves to replace their own faulty valves. I believe it would be a perversion of human sensibility to let infants, young people and men and women die prematurely, out of some bizarre belief that the animal has a greater right to life than the human.

Both before and after I was diagnosed with cardiomyopathy, a lethal heart-muscle degeneration of unknown cause, I argued- and still do-that some medical experiments on animals were cruel and unnecessary. But before surgeons could implant a donor heart into my chest, they had to practice their skills on animals.

Do those opposing xenografts propose that surgeons practice on people instead? Or that they don’t practice at all? Before the surgeons sliced me open with a power saw and cut out my diseased heart, I had to know that I had good odds of awakening.

If my son should ever need the same procedure his father needed, and the disgraceful failure of many Americans to donate organs they no longer need continues to kill people, I hope his doctors will have available the option of saving Ms life with an animal’s heart-either as a bridge to a human heart or as his own new heart. I want surgeons to learn their skills practicing on animals-not on my son.

If an activist needed a heart transplant, would he or she reject the use of an animal heart if that were the only organ available? Would he or she reject even a human heart, on the moral ground that thousands of animals had died to perfect this procedure?

Right now there are about 30,000 Americans waiting for a lifesaving organ transplant. Every day more names are added to this list of desperate people. Among the newcomers are bound to be some of those who carry the protest signs or write the letters. It is one thing to come up with catchy phrases charging animal abuse; it may be quite another to die because your efforts at propaganda have been successful.

Animal organs can help fill the need. True, the medical problems of animal-to-human transplants have not yet been solved. But for at least 10 years, the problems of human-to-human transplants were not solved, either.

It may be tough for these well-meaning people to reverse themselves. But it will be tougher for them to carry their signs outside a hospital where a friend, or the child of a friend, is dying.