The 80-year-old pontiff sought to establish guidelines for rapid 21st-century advances in organ transplants.
He left his summer retreat at Castel Gandolfo outside Rome to speak before an international gathering of 5,000 transplant specialists, a gesture that underscored how important he considers the subject.
John Paul encouraged organ transplants and organ donation, telling the International Congress of Transplant Specialists: "There is a need to instill in people's hearts, especially in the hearts of the young generation, a genuine and deep appreciation of the need for brotherly love, a love that can find expression in the decision to become an organ donor.''
But the address went beyond being a booster speech for organ donations.
John Paul spelled out point by point the church's stand on issues related to transplants: condemning the sale of organs; insisting on informed consent on both sides of the exchange; and singling out the complete end of brain activity as an acceptable way to determine when death has occurred.
Calling organ donation "a genuine act of love,'' he said, "Accordingly, any procedure which tends to commercialize human organs or to consider them as items of exchange or trade must be considered morally unacceptable.''
The decision on who's first in line to receive organs can be based only on medical factors, John Paul said â€“ not a person's age, sex, race, religion, social standing, usefulness to society or any other criteria.
The pope's strong support for organ donations is likely to have an impact on the world's 1 billion Roman Catholics, overcoming traditional Catholic ideas regarding the importance of bodily integrity at death. That concern has meant reluctance among Catholics toward cremation as well as organ donation.
As anticipated, John Paul used the occasion to underline the church's opposition to human cloning.
His speech came within two weeks of Britain's ground-breaking move toward allowing limited human cloning for research. Experts say research there could develop live-saving techniques in a wide range of ailments.
But cloning runs squarely against the church's position that sex between a married couple is the only acceptable way to create human life.
"Methods that fail to respect the dignity and value of the person must always be avoided,'' John Paul said.
"I am thinking in particular of attempts at human cloning with a view to obtaining organs for transplants: These techniques, insofar as they involve the manipulation and destruction of human embryos, are not morally acceptable, even when their proposed goal is good in itself.''
John Paul ruled out use of embryonic cells as well, pointing scientists in the direction of adult stem cells as the acceptable route for research.
Last week, President Clinton praised stem cell research as offering "potentially staggering benefits.'' His praise accompanied a U.S. government announcement of a decision to allow federal funding for research with stem cells that have been removed from human embryos.