The shortage of organs, breakthroughs in research, involvement of biotechnology companies, absence of ethically more acceptable alternatives, and a vaguely perceived "time to put man on the moon" feeling have contributed to the current reawakening of interest in xenotransplantation. The focus of ethical attention has changed from the moral correctness of using animals for research/therapy to an increasingly appreciated danger of the establishment and spread of xenozoonoses in recipients, their contacts, and the general public. The United Kingdom has established an embargo on clinical trials and has set up a national regulatory authority to oversee and coordinate the development of research, establish guidelines, and decide on when trials can proceed. In the United States, on the other hand, the overall attitude is to "proceed with caution," and the Food and Drug Administration has approved a number of xenotransplant studies. The Public Health Service guidelines on reducing infection risk are still evolving and are likely to end up being more cautions than they are currently. There are a number of reasons for not using subhuman primates for xenotransplantation, including their closeness to humans, the likelihood of passing on infections, their depletability (gorillas, chimpanzees), their slow breeding, and the expense of breeding them under specified-pathogen free conditions. The pig, although domesticated and familiar, is too distant to evoke the same feelings we have for primates, has the correct-size organs, is probably less likely to pass on infections, breeds rapidly, and is not endangered; moreover, millions of them are eaten every year. Although drawing ethical conclusions is difficult, at this stage of knowledge and debate it seems acceptable to manipulate pigs genetically and to proceed to using their organs for xenotransplantation trials when infection control measures and the scientific base justify it. The question of informed consent is likely to be a vexing one. It might end up more of a binding legal contract than consent as we understand it now. Xenotransplantation is also unlikely to cost less than, or significantly alleviate the shortage of, cadaveric organs in the short term. The international dimension of the risk of infection is becoming obvious, but there has so far been no effort to convene an international forum to agree on universally acceptable guidelines.