Doctors in New York City have transplanted a pig's organ into a brain-dead man and watched as it performed normally for over a month.
Brain death is a state in which a person's brain no longer carries out even its most basic activities for life.
The pig kidney transplant is a step toward realizing the hopes of the New York team to try the operation on living patients.
Scientists around the country want to learn how to use animal organs to save human lives.
Doctors believe bodies donated for research will help them develop the operation for patients.
The latest experiment was announced Wednesday by New York University Langone Health.
It marks the longest time that a pig kidney has worked in a living or dead person.
And the experiment is not over.
Researchers will follow the kidney's performance for a second month.
"Is this organ really going to work like a human organ? So far, it's looking like it is," Dr. Robert Montgomery told The Associated Press.
He is the director of NYU Langone's transplant institute.
"It looks even better than a human kidney," Montgomery said on July 14.
On that day, he replaced a dead man's own kidneys with a single kidney from a genetically engineered pig.
He watched the replaced pig kidney immediately start producing urine.
The possibility that pig kidneys might one day reduce the shortage of transplantable organs persuaded the family of Maurice "Mo" Miller to donate his body.
He had died suddenly at 57 with a formerly undiagnosed brain cancer, which did not permit for usual organ donation.
"I struggled with it," his sister, Mary Miller-Duffy, told the AP about her decision.
But he liked helping others and "I think this is what my brother would want. So, I offered my brother to them."
"He's going to be in the medical books, and he will live on forever," she added.
Attempts at successfully performing animal-to-human transplants, or xenotransplantation, have failed for many years.
Often, the human immune system attacks the foreign tissue.
Now researchers are using pigs genetically engineered so their organs are a better fit for human bodies.
The Food and Drug Administration (FDA) is considering approval of some small but detailed studies of pig heart or kidney transplants in volunteer patients.
More than 100,000 patients are on the United States' transplant list.
The University of Maryland's Dr. Muhammad Mohiuddin warns that it is not clear if a dead body reacts the same way as a live body to a pig organ.
But he said the research educates the public about xenotransplantation, so "people will not be shocked."
Before this most recent experiment, NYU and a team from the University of Alabama at Birmingham tested a pig kidney transplant in a dead body for just two or three days.
An NYU team has also tested transplanted pig hearts on donated bodies for three days of intense testing.
But how long should these experiments last?
University of Alabama's Dr. Jayme Locke said that it is not clear.
Among the moral questions is how long a family can emotionally deal with it.
Because keeping a body alive after the brain has died is difficult.
In her own experiment, the donated body was stable enough that if the study wasn't required to end after a week, "I think we could have gone much longer, which I think offers great hope," she said.
I'm Gregory Stachel.