New York researchers transplanted pig hearts into two brain-dead people over the last month, the latest in a string of developments in the long quest to one day save human lives with animal organs.

The experiments announced Tuesday come after a historic but failed attempt earlier this year to use a pig's heart to save a dying Maryland man — sort of a rehearsal before scientists try again in the living.

Among the lessons: Practice with the deceased is important.

"We learned so much from the first one that the second one is much better," said Dr. Nader Moazami, who led the operations at NYU Lang...