Two chimera piglets containing monkey DNA have been born in China.
Although both died within a week and appeared to be normal, the baby animals had genetic material from cynomolgus monkeys in their heart, liver, spleen, lung and skin.
Scientists said the research, which required more than 4,000 embryos to get the piglets, aims to find ways of growing human organs in animals for transplantation.
'This is the first report of full-term monkey-pig chimeras', Tang Hai at the State Key Laboratory of Stem Cell and Reproductive Biology in Beijing told New Scientist.
Five-day old piglet embryos had monkey stem cells injected into them that had been adjusted to produce a flourescent protein, allowing researchers to find out where the cells ended up.
The scientists said it was unclear why the two chimera piglets died, but as eight other normal piglets that were implanted also died, they think this is a problem with the IVF process rather than chimerism.
Despite the research, some members of the scientific community have warned against creating chimeras due to ethical concerns.
Neuroscientist Douglas Munoz at Queen's University in Kingston, Canada, said that research projects like this 'just really ethically scares me'.
'For us to start to manipulate life functions in this kind of way without fully knowing how to turn it off, or stop it if something goes awry really scares me.'
However, China shows no sign of stopping after proposing in July to create monkeys with partially human-derived brains in order to better study diseases like alzheimer's.
And Yale University stem cell expert Alejandro De Los Angeles has written that the search for a better animal model to stimulate human disease has been a 'holy grail' of biomedical research for decades.
'Realising the promise of human-monkey chimera research in an ethically and scientifically appropriate manner will require a coordinated approach', he said.
A human-pig hybrid embryo was created in January 2017, at the Salk Institute in San Diego, but died 28 days later.
It is hoped the research could offer an alternative to organ donation.
Around three people a day die in the UK according to the NHS and 12 in the US because replacement organs cannot be found.