A 36-year-old man born without testicles received one transplanted from his identical twin brother in a six-hour operation performed on Tuesday in Belgrade, Serbia, by an international team of surgeons.

The surgery was intended to give the recipient more stable levels of the male hormone testosterone than injections could provide, to make his genitals more natural and more comfortable, and to enable him to father children, said Dr. Dicken Ko, a transplant surgeon and urology professor at Tufts University School of Medicine in Boston, who flew to Belgrade to help with the procedure.

The operation was only the third known transplant of this type. The first two were performed 40 years ago in St. Louis, also for identical twins, each pair with a brother lacking testicles.

The absence of testicles is an exceedingly rare condition, but doctors say that the surgery may have broader applications for transgender people, accident victims, wounded soldiers and cancer patients. But the procedure raises questions about the ethics of transplants that are not lifesaving, and about the possibility of recipients’ someday fathering children with sperm from donors who may not even be related to them.