In news that may hearten vampires worldwide, medical researchers have discovered that a dose of youthful blood can rejuvenate an aging heart—at least in mice. Specifically, a protein called growth differentiation factor 11 (GDF11) found in the blood of young mice can reverse a common heart condition in the elderly called cardiac hypertrophy, which often leads to heart failure. To investigate the antiaging compound, a team of researchers led by Amy J. Wagers and Richard T. Lee at the Harvard Stem Cell Institute connected the circulatory systems of old and young mice so that they shared the same blood. After four weeks of sharing the same circulatory system, the cardiac hypertrophy in the old mice “dramatically regressed,” they report in Cell (2013, DOI: 10.1016/j.cell.2013.04.015). The team used proteomics methods to identify the rejuvenating protein in young mouse blood that is lost with age. Finally, to confirm that GDF11 was the youthful elixir and to rule out other sources of age reversal, the researchers injected the protein directly into old mice and saw the same rejuvenating effects. If the protein works the same way in humans as it does in mice, the discovery may eventually lead to new heart therapies.