"Organ Farm" visits several experimental facilities where pigs are being genetically-modified for use in humans. Because a human body would immediately reject a pig organ as foreign, these "transgenic" pigs are genetically altered with human DNA in the hope that a human recipient's body will be fooled into thinking the organ is human. The end goal is to create a living production line of these partially humanized pig organs to use as spare parts for humans.

And, while such whole-organ xenotransplants are still in the future, testing already is underway on fetal pig cell treatments for brain impairments like Parkinson's disease, stroke, epilepsy, Huntington's disease and spinal cord injuries. FRONTLINE follows several patients in these experimental trials, including a twenty-one-year-old stroke victim and a man incapacitated by severe Parkinson's disease. Xenotransplantation was their last hope.

This view of medicine's possible future, seen from the inside, also includes interviews with top xenotransplant doctors, researchers and medical ethicists, many of whom discuss not only the great promise of this breakthrough science, but also speak candidly about its risks.

Because pig cells and organs contain a unique virus within their genetic material, some fear that transplant recipients would not only contract this virus--known as Porcine Endogenous Retrovirus, or PERV--but also possibly spread it among the general population. Some scientists also worry about the possibility of still unknown viruses which could be transmitted in xenotransplantation.

"The ultimate concern is that you create AIDS II by doing xenotransplantation," says Hugh Auchincloss, surgical director for transplantation at Massachusetts General Hospital and chairman of the FDA's subcommittee on xenotransplantation. "And nobody is quite capable of saying that's impossible."

The development of xenotransplantation is also under attack by animal rights' groups, which have denounced the extensive experimentation on wild primates and pigs in the name of scientific advancement. In "Organ Farm," FRONTLINE goes behind the scenes at facilities conducting the controversial animal experiments, giving viewers the chance to view the experiments in progress. Surprisingly, scientists engaged in xenotransplant research do not completely refute the charges made by animal rights activists.

"We have to be frank about this: We are exploiting these pigs," says Dr. David White, director of research at Imutran in Cambridge, England. "But I believe it's far more justifiable to exploit these pigs in order to save people's lives than for the production of food."

It's a debate that has both sides frustrated--particularly doctors and researchers, who say that every moment spent arguing over the ethics of animal experimentation delays a scientific breakthrough that could revolutionize medicine and save millions of lives.