Pumping the blood of young mice into the diseased hearts of old mice rejuvenated those organs, a new study found—raising the possibility that a similar technique will work in humans. As people and mice get older, their heart muscles thicken and sag, becoming less effective at pumping blood and sometimes failing altogether. But when Harvard University researchers surgically connected the blood stream of a young mouse with that of an old mouse, they found that the infusion of the young mouse’s blood dramatically changed the old mouse’s heart. “It was quite striking how the heart responded,” study author Amy Wagers tells New Scientist. Within a few weeks, it became “pretty much the same” as the heart of the younger mouse, and the infusion of older blood didn’t seem to hurt the young mouse at all. Researchers found that the young mouse’s blood contained high levels of a protein called GDF11. Simply injecting the protein into old mice, they found, also rejuvenated their hearts. Human blood also contains GDF11, and researchers now plan to investigate whether it could reverse the effects of aging on human hearts, too.