Over the last 20 year we have been exploiting a broad host range, lytic bacteriophage (D29) as a DNA lysing agent to develop rapid and sensitive tests to detect and identify mycobacterial pathogens. This work has led to a method that can detect specific species of Mycobacterium within 6 h, and this has been commercialised as the Actiphage test. During these studies we have discovered new features of the phage-host interaction that has allowed us to better understand and improve our test methods and we have shown that the method can be used to diagnose mycobacterial infections in a range of animals, including exotic species such as lions. However, for a new biotech product to flourish there has to be a commercial market, and recently this led us to ask whether Actiphage can also be used to detect TB in human blood and produced some surprising results. These studies have once again demonstrated the value of phage as a biotechnological tool and how they can reveal new things about these difficult to study bacteria.