The article deals with an important figure in forensic biology, Paul Uhlenhuth, who was the first to discover a method for distinguishing between human and animal blood. The article describes his life and the context that led him to criminology. His contribution to forensic biology was partly a coincidence. As a doctor, he was involved in the development of drugs against foot-and-mouth disease and, quite by chance, he came to use these medical methods in forensics. The article describes a groundbreaking case in which Uhlenhuth's method of distinguishing between human and animal blood was used.