It was the fourth pig kidney transplant in the United States, and the first of three that will be done at Mass General as part of a new clinical trial sanctioned by the Food and Drug Administration. Two of the previous patients died shortly after the procedure, including one who was critically ill before the transplant.

The surgery comes at a turning point in the quest to tell if animal-to-human transplants could help ease the shortage of donated human organs.

The recipient was Tim Andrews, a 66-year-old New Hampshire resident who had end-stage kidney disease and was on dialysis for more than two years.