Up until quite recently, the formidable problems of cross-species rejection made xenotransplants generally impractical. But that is being overcome, and many scientists believe that we are on the verge of an explosion of xenotransplants. The benefits will be enormous. Patients will not have to wait endlessly for suitable donors. And apart from solid organs - heart, liver and kidney - implants of animal cells and tissues will bring benefits to much greater numbers of patients. Xeno cells and tissues could be used to cure diabetes and treat Parkinson's and Huntington's disease, multiple sclerosis and the effects of strokes.

But the other side of the equation is equally awesome. There is a small but real risk of xenozoonosis, the transmission of animal diseases to humans via organ transplants or blood.

The child's chest was opened and her heart removed. The implantation of the animal heart took about one hour, after which the child's blood temperature was returned to normal, her new heart started and her body weaned slowly from the heart-lung machine, according to the hospital.

Hospital spokesmen said the operation profited from experience in the transplant of ape organs, including kidney xenotransplants performed in the early 1960s and the use of baboons in humans with liver failure.

It is becoming customary to speak of allotransplants and xenotransplants instead of homotransplants and heterotransplants.