The campaign group compares the work's secrecy to that which surrounded gene therapy until the death of a patient, Jesse Gelsinger, during clinical trials at the University of Pennsylvania last year. Alix Fano, director of the group, says that it wants xenotransplantation research to be as open as gene-therapy trials now are. "This is a new form of biotech — it's akin to the gene-therapy trials where a lot of side effects went unreported," she says.

Fano estimates that there are around a dozen xenotransplantation trials under way in the United States. The precise number is unknown, she says, because the FDA does not disclose it. The trials began more than 30 years ago, with about 12 transplants of chimpanzee organs. More recent experiments have studied the transplantation of cells, rather than complete organs.

LeRoy Walters, director of the Kennedy Institute of Ethics at Georgetown University in Washington DC, supports the idea that data from medical research should be more freely available. "I would like to see a public registry of all clinical trials across the board," he says.

The lawsuit reflects the tension between the industrial sponsors of research, who want to keep the results of clinical trials private, and some public-health advocates, who want more openness. That tension increased last autumn following Gelsinger's death, which set in motion a chain of events that resulted in a publicly accessible database of gene-therapy clinical trials.

Drug companies say that the release of information from clinical trials can threaten their competitiveness. But campaigners and bioethicists argue that both gene therapy and xenotransplantation differ so much from conventional medicine that more transparency is needed.

Gene therapy, they say, runs the risk of germline gene transfer, where genetic changes are passed unintentionally to the patient's children. With xenotransplantation, critics worry that retroviruses might pass from animals to humans.