The momentum towards clinical trials of xenotransplantation is seemingly unstoppable, powered as it is by new prowess in genetic engineering that promises to pulverize the barrier of cross-species rejection, blended with an otherwise insatiable need for transplantable organs and multimillion dollar investment by biotechnology companies (see page 320).

But overcoming the huge obstacles to cross-species rejection is much further off than some biotechnology companies would like their investors and the public to believe. And premature trials carry the risk of creating new human diseases and pandemics (see page 327). Regrettably, the debate about xenotransplantation has amounted to a cacophony of messages from the various stakeholders about the potential risks and benefits. How can the public be expected to evaluate the possibilities when the experts themselves cannot agree?

Regulation of human cloning is politically easy, given its restricted benefits and the practical difficulties. Much greater political courage and enlightenment will be required to clarify the complex and pressing question of how best to regulate xenotransplantation, and this time in the face of predictable opposition from industrial and patient constituencies. Problems must not be papered over through compromises between the various conflicting interests, brokered by bureaucrats and expert committees within regulatory agencies, as has been amply shown by the bovine spongiform encephalopathy crisis, and the contamination of haemophiliacs with HIV during the 1980s.

A well-organized and informed public debate should precede any action by regulatory agencies. One model for the latter, offered on page 326, deserves studious attention. But until action is taken in this direction, politicians would do well to err on the side of caution, and agree on an international moratorium on clinical trials. What is at stake is not only risks to public health, but the real promise of xenotransplantation, which can only be compromised by undue haste.