Hikers discovered "Oetzi the Iceman" with an arrow buried in his back, and researchers had already determined he died from his wounds.
The frozen body is remarkably preserved, and scientists writing in the Journal of the Royal Society say that preservation extends to the bleeding that occurred before his death, the BBC reported Wednesday.
Blood cells degrade quickly, and earlier searches for blood within the frozen body showed no signs of blood cells, researchers said.
Albert Zink and colleagues at the Eurac Institute for Mummies and the Iceman in Bolzano, Italy, have now used a technique known as atomic force microscopy to examine thin slices of tissue taken from an area around the arrow wound and discovered structures with the tell-tale "doughnut" shape of red blood cells.
They are by far the oldest red blood cells ever observed, researchers said.