Scientists are almost certain an old piece of cloth contains the blood of King Louis XVI of France, who was guillotined in Paris on 21 January, 1793.
They say DNA on the cloth is remarkably similar to some from the mummified head of an earlier King from the 16th Century.
After Louis XVI was beheaded, many spectators dipped their handkerchiefs in his blood.
The BBC reports the piece of fabric was placed in a dried gourd, decorated with images of revolutionary heroes and the words:
"On January 21, Maximilien Bourdaloue dipped his handkerchief in the blood of Louis XVI after his decapitation".
The souvenir has been in the hands of an Italian family for a century.
A team of experts from Spain and France has published its findings in Forensic Science International journal.
The team found a rare genetic signature shared with DNA from a mummified head belonging to King Henri IV, who was killed in 1610.
In 2010, scientists said the head belonged to Henri, saying it shared the same physical features as those illustrated in contemporary portraits of him.