Spanish police say they have arrested five people suspected of trying to force a Moroccan immigrant to sell one of his kidneys for 6,000 euro (£4,300).
A police statement said the man tried to pull out of the deal while undergoing clinical tests but the group kidnapped him and threatened to kill him if he did not go ahead.
The statement said the alleged buyer was a leader of a criminal gang whose son had kidney problems.
Spain's National Transplant Organisation alerted police to the case.
The statement said the gang, which burgled luxury houses in Spain and abroad, was dismantled with the help of Belgian and German police over the past two years.
Fourteen European nations signed an international treaty to fight human organ trafficking in Spain in March.