In 1667, in a small Parisian chamber lit by oil lamps and crowded with curious observers, a young physician named Jean-Baptiste Denys carried out an experiment that would ignite debate across Europe. With a goose-quill tube, a silver cannula, and the blood of a gentle lamb, Denys performed what is widely regarded as the first documented transfusion of blood into a human being. It was a moment of bravery, ingenuity, and—by today’s standards—remarkable risk, but it also marked a milestone in the long and troubled history of transfusion medicine.
To understand the audacity of Denys’s procedure, it is necessary to look back at the ideas, ambitions, and early experiments that shaped the 17th-century scientific imagination.
Jean Denys performing a xenotransfusion from dog to man. Credit: Johannes Scultetus’ Armamentarium chirurgicum (1693)
For centuries, blood was viewed less as a biological fluid and more as a mystical essence—life, vitality, courage, or temperament. Medieval physicians believed illness came from imbalances in the four humours, not from anything measurable in blood. As a result, bloodletting—rather than transfusion—became the most common intervention.
In 1628, English physician William Harvey published De Motu Cordis, describing the circulation of blood. With this revelation, blood became something that could be measured, manipulated, and potentially replaced. Harvey’s insight unlocked the possibility that blood might carry transferrable qualities—nourishment, heat, or even character.
Early Animal Experiments
By the 1660s, natural philosophers at the newly formed Royal Society in London and the Accademia del Cimento in Florence were pushing the boundaries of experimental physiology. They performed transfusions between animals, usually dogs, by connecting their blood vessels with quills or brass tubes. These experiments showed that animals could survive the passage of blood from one to another, at least briefly.
In Florence, Giovanni Alfonso Borelli studied circulation mechanically. In Oxford, Christopher Wren injected animals with various experimental solutions using quills. And in London, Richard Lower, a brilliant physician associated with the Royal Society, performed the first successful transfusion between dogs in 1665.
These early successes suggested a radical possibility: if animals could exchange blood, perhaps humans could too.
Jean-Baptiste Denys
Jean-Baptiste Denys was not yet 30 when he was appointed personal physician to King Louis XIV. Born around 1640, he trained not only in medicine but also in mathematics—an unusual combination that aligned him with the experimental spirit sweeping through Europe.
Denys watched the English experiments closely. The Royal Society had begun circulating letters describing their dog-to-dog transfusions, and he believed France should not fall behind.
Denys started collaborating with the barber-surgeon Paul Emmerez to undertake blood transfusion. In one of his first recorded cases, he transfused nine ounces of blood from one dog to the other. The experiment was terminated only when one of the dogs took ill suddenly. In a later experiment, Denys noted that the dogs behaved predictably with no changes in eye movement, food consumption, and the weights of the subjects.
Emboldened by the success with the dogs, Denys decided that he could take the next leap: transfusing blood from animals into humans. He reasoned that cross-species transfusion from a gentle and docile creature, such as a lamb, was not only safe but it might even cure certain illnesses, soothe agitation and restore vitality.
Denys’ vison went against the ideals of the Academy of Sciences of Paris and Faculty of Medicine, and so he took his research to the private academy established by scholar Henri Louis Habert de Montmor, who saw an opportunity to surpass both the English and the conservative French Academy of Sciences and consequently gain his own glory.
The opportunity came in June 1667. A fifteen-year-old boy, wracked by two months of fever and weakened by excessive bloodletting, was brought to Denys. Using a goose-quill tube and rudimentary surgical connectors, Denys transfused around 12 ounces of lamb’s blood into the boy. The boy not only survived the ‘treatment’, his conditions remarkably improved.
Jean-Baptiste Denys performs the first human transfusion. Credit: Wikimedia Commons
News travelled quickly. Europe buzzed with the rumour that France had accomplished what England dared not. For a brief moment, Denys became a star.
Buoyed by success, Denys performed several more transfusions. The second transfusion, conducted on a middle-aged butcher, was equally successful. The third patient fared badly: he died shortly after having a transfusion. It was Denys’ fourth patient that finally undid him.
The Death of Antoine Mauroy
Antoine Mauroy was a man who was suffering from violent mental disturbances. Denys believed Mauroy’s agitation could be softened by transfusing the mild “temperament” of calf’s blood.
According to the Wikipedia article on Jean-Baptiste Denys, Mauroy was “abducted from the streets of Paris by Montmor's guard and tied to a chair and transfused with blood in front of an audience of noblemen.” I couldn’t verify the authenticity of this seemingly bizarre way of acquiring subjects, but whatever the circumstances, Mauroy became Denys’ final transfusion patient.
In the hours following the procedure, Mauroy experienced a debilitating fever, nausea, diarrhea, nosebleeds, and urine that was as black as 'chimney soot', fever, tachycardia, and abundant sweating. Today these are recognized as classic signs of acute haemolytic transfusion reactions, triggered when a recipient’s immune system destroys incompatible blood. But a few days later, the man had apparently fully recovered. This was the final proof for Denys, who immediately publicized his success.
Mauroy and his wife Perrine eventually returned to their modest home, but Perrine soon found out that her husband's newfound calmness was temporary, lasting only two months. The man's state of health and mind changed abruptly due to his binges of wine, tobacco, and 'strong waters' (alcohol). The man's madness was worse than before.
Denys performed a second transfusion which diminished the delirium but induced other major side effects. On the insistence of Mauroy’s wife, Denys prepared for a third infusion but before he could inject more blood, Mauroy's body shook in a 'violent fit', at which Denys decided to end the transfusion. Mauroy died the next day.
Mauroy’s widow accused Denys of murder, claiming he had killed her husband with his strange procedures. Denys was convinced his procedure was safe and insisted that this trial was rather a consequence of his decision to pursue research against the will of the King's Academy of Sciences as well as that of the major players of the conservative Parisienne Faculty of Medicine.
An inquest was held where it was discovered that Mauroy's widow had, allegedly, persuaded and offered large amounts of money by several "unknown" physicians, to bear false witness and file reports against Denys' blood transfusion experiments. A police investigation revealed that Mauroy had actually been poisoned by his wife.
Denys was cleared of all accusations, but the judge ordered that "no transfusion should be made upon any human body but by the approbation of the physicians of the Parisian Faculty (of Medicine)", forcing Denys to end his studies in blood transfusions.
Finally, in 1670, the Royal Society and the French government formally banned transfusion. Even the Catholic Church weighed in with prohibitions. The Royal Society in London also abandoned the field.
For almost 150 years, no one dared to revisit the idea.
Renewed Interest
Transfusion resurfaced only in the early 19th century, when British obstetrician James Blundell used human-to-human transfusion to treat postpartum haemorrhage. But it was Karl Landsteiner’s discovery of the ABO blood groups in 1900 that made transfusion reliably safe. Once blood types could be matched, the haemolytic reactions that killed Mauroy were finally understood.
From that point forward, transfusion medicine evolved rapidly—blood banks, anticoagulants, refrigeration, plasma separation, and the modern logistics that support surgeries and emergency care worldwide.
Today, blood transfusions are a critical component of medical treatment, having saved millions of lives worldwide.
Jean-Baptiste Denys died in 1704. After the trial, he returned home and resumed teaching. Four years later, he invented styptic, an antihemorrhagic liquid, that is now commonly used around the world to staunch bleeding.
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