A lack of human donors has long pointed to the need for animal pancreatic tissue in such transplants. But earlier studies using xenotransplants to treat diabetes failed to show clear benefits. And none used the encapsulation technique, which spares clusters of transplanted pig cells, which are the size of grains of sand, from the ravages of the host immune system.

"This is the main thing, "Elliot says, "protecting the transplanted cells from this double whammy that comes from rejection of foreign tissue and the diabetes itself, in which the body attacks its own pancreatic tissue."

Elliot says that the pigs used for the transplants were bred and kept in an exceptionally sterile environment, which meant they were "free of viruses, bacteria, and parasites."

In addition, he says tests have shown that retroviruses hidden away in the pigs' genes are not capable of producing viable viral particles. The discovery of such viruses in pigs in the late 1990s has been a key factor holding back xenotransplantation.

Anthony d'Apice, director of the Immunology Research Centre of St Vincent's Hospital Melbourne, says demonstrating a good risk-to-benefit ratio will decide whether the trial is successful. "It's about minimizing the risk of failure--and the risk of infection, which is not just a risk for the patients but also their contacts and the community, and this may be from known or unknown organisms including porcine endogenous retroviruses."

Jonathan Stoye, a virologist at the U.K.'s National Institute of Medical Research and an advisor to the British government on xenotransplant safety, backs the study. "The risk from these porcine viruses is probably much lower than we feared at first--although you can never completely discount it," he says. "I would like to see the clinical data before I draw any conclusions, but it seems to me to be a very good trial to be doing."