Although it appears that primates will never be used as sources for human transplants, they are being used in xeno research, primarily as recipients for pig organs. In the UK such experiments are permitted where the researchers can demonstrate that there is no alternative. The reasoning is that while using an ape or monkey as an organ source could potentially benefit just one, or at most a handful of humans, the amount of knowledge that could be gained from research on just one animal could potentially help many people.

Given the closeness of man to non-human primates, apes are used very sparingly, though experiments are regularly conducted on monkeys. In 1999 the Home Office, the department which licenses animal experiments in the UK, revealed that 270 monkeys had been killed in xenotransplantation research in the last four years. In 1999 three baboons were used in xeno research and seventy-nine cynomolgus monkeys. Primates are therefore used - though in limited numbers - in both the UK and the US. In particular, pigs' hearts are grafted onto baboons to test and monitor the efficiency of the anti-rejection measures undertaken by the scientists.

By any standards, these experiments are some of the most grisly procedures carried out anywhere in the name of science. They do sometimes involve a full transplant of a genetically modified pig heart into a monkey. In some cases, however, the doctors will graft the transgenic hearts onto a baboon's neck arteries, as this allows them to observe the way the pig heart behaves in another species, and monitor the rejection process. The operation is carried out under general anaesthetic and the baboon is humanely killed afterwards. These measures, however, do not pacify animal rights campaigners, who say the experiments are cruel and unnecessary.

Some of the fiercest criticism of animal experiments comes in Britain, where the animal rights debate has a long history - the world's first anti-vivisection organisation was founded in London in 1875. At the heart of the animal rights movement today is the British Union for the Abolition of Vivisection (BUAV). Founded in 1898, the organisation campaigns against any use of animals in medical experiments. Their general case against animal experiments is that it is morally unjustifiable. 'To inflict suffering on defenceless animals during experiments is wrong. We do not have the right to experiment on or use animals for our benefit,' they say.

However, they have additional specific arguments against xenotransplantation. They point to the immunological, physiological and biochemical differences between pigs and humans as challenges yet to be overcome.

Dr. Gill Langley is not what some would regard as a typical animal rights campaigner and her opposition to animal experiments is based on far more than an emotional attachment to furry creatures. She has a PhD from Cambridge University's zoology department, is a Fellow of the Royal Society of Medicine and has been a member of the Animal Procedures Committee since 1998. She is a scientific adviser to the BUAV and the Dr. Hadwen Trust for Humane Research, which researches into alternatives to using animals in medical research.

'Even for people opposed to all animal experiments, xenotransplantation research is a particular cause for concern. The point about any new research area like this is that it is going to grow. In particular, it will use more and more animals - either as the research progresses or if the technique is ever perfected. This is a direct contradiction of the general trend, which is to reduce the number of animals used in scientific procedures.'

Though they welcome the fact that primates will never be used as source organs, the BUAV say the numbers of primates used in experiments is indefensible. 'There has been a move to cut down on the number of primates in all areas of medical research, so the number used in xeno procedures is very high by comparison. In addition, the scientists are encountering new problems with the research all the time. This is a terrible waste of animal lives,' says Dr. Langley.

The BUAV claim that primates used in xenotransplantation research will experience a large number of traumatic procedures, including major surgery, from which many will die; internal haemorrhages; isolation in small cages; repeated blood sampling; wound infections; nausea, vomiting and diarrhoea because of immunosuppressant drugs and kidney or heart failure.

'It's not just the suffering they endure in the laboratories and research establishments,' Dr. Langley explains. 'Just getting there can be torture. Studies of primates show them to have complex mental abilities which may increase their capacity to suffer. Supplying the laboratories in the UK imposes huge suffering on the animals. It involves capturing wild individuals, usually in Africa. They're then contained in small, single cages, and transported for very long distances causing deaths, distress and suffering. A number of inquiries have said that the use of primates was unacceptable and should be limited to very small numbers. Our view is that any number is unacceptable.'

Critics also claim that the animals are treated cruelly and the experiments are still a long way from succeeding - both claims fiercely denied by the companies involved.

Xeno researchers say they are forced to use primates for some of the research because of their genetic similarity to humans. They say that all humans have pre-programmed antibodies against pigs, and only apes and certain types of monkey have the same immune system as us. Therefore, only these animals can demonstrate how a human might react to a pig organ.

In the UK scientists have had their homes firebombed by animal rights organisations whose views and supporters are more extreme than the BUAV, and regularly receive death threats. In public they say little for fear of attracting more attacks, but Dr David White, whose research at Imutran has been crucial to modern xenotransplantation, defends the work staunchly.

'As far as the animal rights activists are concerned, I accept that they have a fight to reject animal experimentation. I accept they have a fight to reject for themselves the product of medical research on animals. But I don't accept that they have a fight to prevent the population at large from benefiting from what that research can bring. And even an animal rights activist, when he's sick, goes to the doctor and says "Help me".

'If the animal rights people had their way, they would stop all progress in medical research. There is not a single medical advance that's currently being placed in hospitals that hasn't gone through animal research. You stop animal research, you stop medical advance, it's as simple as that. I find that an unacceptable position for anybody to take up.

'The regulations under which we conduct our experiments here in the UK are the strictest anywhere. Everything we do is licensed by the Home Office. Their inspectors regularly come and watch what we do and comment on the way that we do it. Not only am I satisfied that the animals' suffering is kept to a minimum, the Home Office inspectorate are also satisfied.'

In the US, doctors and scientists working in this field express their views even more openly. In an operating theatre in Ohio, doctors carry out experiments on monkeys similar to those performed in the UK. Looking down on them from the wall is a poster. It shows a photograph of a sick child, with the caption, 'The animal rights message to someone suffering from a deadly disease - tough luck!'' The differences between the two sides in this debate are clearly irreconcilable.

Few of the arguments against using primates for source organs can be made against using pigs. Man and pigs have lived together for thousands of years and pigs were among the first animals to be domesticated. Cave paintings of boars survive from ice age Europe, dating from between 35,000 and 20,000 BC. In the Bible, Noah took two pigs onto the Ark to escape the Flood, which scholars estimate happened about 13,600 BC.

Charles Darwin's On the Origin of Species claims that the Chinese had domesticated pigs by 7000 BC and swine feature regularly in Greek literature. Closer to home, at the time of the Domesday Book the size of some settlements in Surrey and Sussex was calculated by the number of pigs they sustained.

We haven't only lived alongside pigs - there is a long history of using parts of pigs, including tissue, to help sick humans. Pig heart valves have been implanted into humans for more than twenty years, and since the 1930s pig insulin has helped millions of people with diabetes live more normal lives. Pig skin grafts have been used for burns victims for many years. This long history, say supporters of the xeno project, illustrates that there has been ample opportunity for men to contract dangerous pig diseases, but that very few have become apparent.

Despite all this close contact, nature can still spring surprises. In 1999 a virus never seen before in pigs infected a large number of pigs in Malaysia. At least 117 people who worked with pigs died. More than a million pigs were slaughtered to try to contain the virus, and thousands of people fled the area.

At a practical level, pigs seem to offer the most hopeful option as source animals because they are anatomically and physiologically similar to humans. Their kidneys, hearts, lungs and livers work in a broadly similar way to humans'.

There are estimated to be about 7,000 million pigs in the world at any one time. When a pig is born it's about the size of a man's hands. Within five months it's torso is as big as a man's. At this stage a pig's organs are the optimum size for transplanting into humans. This is an important advantage of the pig as a potential organ source - it's the right size.

Sheep and goats are too small and cows are too big. There are also fewer ethical objections in using animals already bred in captivity for food. 'It cannot be right to breed pigs to make people fat, but not to save their lives,' says one researcher. 'Any objections to using pigs for medical procedures should start at the breakfast table, not the operating table,' says another.

The ethical arguments against using primates for xenotransplantation are therefore not as strong when applied to pigs. In the balance of the rights of the animal to be free from harm versus the rights of humans potentially to live longer or better by exploiting it, the pig has fewer supporters than the ape or monkey.