A 57-year-old man with life-threatening heart disease has received a heart from a genetically modified pig, a groundbreaking procedure that offers hope to hundreds of thousands of patients with failing organs.

It is the first successful transplant of a pig’s heart into a human being. The eight-hour operation took place in Baltimore on Friday, and the patient, David Bennett Sr. of Maryland, was doing well on Monday, according to surgeons at the University of Maryland Medical Center.

“It creates the pulse, it creates the pressure, it is his heart,” said Dr. Bartley Griffith, the director of the cardiac transplant program at the medical centяer, who performed the operation.

“It’s working and it looks normal. We are thrilled, but we don’t know what tomorrow will bring us. This has never been done before.”

Scientists have worked feverishly to develop pigs whose organs would not be rejected by the human body, research accelerated in the past decade by new gene editing and cloning technologies. The heart transplant comes just months after surgeons in New York successfully attached the kidney of a genetically engineered pig to a brain-dead person.

Researchers hope procedures like this will usher in a new era in medicine in the future when replacement organs are no longer in short supply for the more than half a million Americans who are waiting for kidneys and other organs.

“This is a watershed event,” said Dr. David Klassen, the chief medical officer of the United Network for Organ Sharing and a transplant physician. “Doors are starting to open that will lead, I believe, to major changes in how we treat organ failure.”

Mr. Bennett decided to gamble on the experimental treatment because he would have died without a new heart, had exhausted other treatments and was too sick to qualify for a human donor heart, family members and doctors said.

His prognosis is uncertain. Mr. Bennett is still connected to a heart-lung bypass machine, which was keeping him alive before the operation, but that is not unusual for a new heart transplant recipient, experts said.

The new heart is functioning and already doing most of the work, and his doctors said he could be taken off the machine on Tuesday. Mr. Bennett is being closely monitored for signs that his body is rejecting the new organ, but the first 48 hours, which are critical, passed without incident.

He is also being monitored for infections, including porcine retrovirus, a pig virus that may be transmitted to humans, although the risk is considered low.