Embryonic stem cells, hailed as a potential treatment for a range of diseases, are contaminated by an animal molecule, researchers suggest.
Research using embryonic stem cells is controversial
The molecule's presence would lead the human immune system to attack the cells.
The University of California team, writing in Nature Medicine, says this means such cells could not be used.
They say the only solution is to create new stem cell lines without using animal-derived products.
Human embryonic stem cells are taken from embryos created, but not used, in fertility treatments.
These cells can potentially generate every body cell type.
Once they have been removed, they are usually cultured with animal-derived 'serum replacements', which are sources of sialic acid Neu5Gc, a molecule against which many humans have circulating antibodies.
Neu5Gc is found on the surface of animal cells, but not on human cells and is therefore attacked by the human immune system.
It is the reason why the human body rejects organs transplanted from animals.
The Neu5Gc molecule is taken up by stem cells when they are grown in laboratory cultures that contain animal-derived materials, including so-called "feeder layers" from mice and foetal calf serum.
The study, led by Dr Ajit Varki at the Departments of Medicine and Cellular and Molecular Medicine at the University of California, found embryonic stem cells were contaminated with substantial amounts of Neu5Gc.
Dr Varki said: "The human embryonic stem cells remained contaminated by Neu5Gc even when grown in special culture conditions with commercially available serum replacements, apparently because they are derived from animal products.
"It would seem best to start over again with newly derived human embryonic stem cells that have never been exposed to any animal products."
However, he said regulations in the US which only allows federal funding to be given to research on embryonic stem cell lines created before 2001 would mean such an approach could not be pursued without a change in the law.
Dr Stephen Minger of King's College London, who created the first embryonic stem (ES) cell line in the UK, told the Times newspaper: "My perspective has always been that, when we are serious about getting ES cell therapies into the clinic, we will have to derive new lines completely free from animal conditioning."
Patrick Cusworth, a spokesman for the pro-life charity Life, said: "If anything, this latest report has demonstrated the sheer unpredictability of embryonic stem cells, and the enormous potential hazards if these were implanted into human patients."