Two methods of using the radioactive gas krypton 85 for leak detection are described. In the first, components to be tested are immersed in the gas, after which they are removed and individually monitored for radioactive content. Application of the method to detection of leaks in transistor encapsulations is discussed. Correlation has been obtained between leak rate and time for subsequent electrical failure.The second method of using krypton 85 is to insert a small quantity into each component before the final seal is made. Leak rates are then measured by collecting gas which leaks out and assaying it with a scintillation counter. This technique is illustrated by tests carried out on a hermetically-sealed pressure switch. The method is compared with the helium mass spectrometer, and it is concluded that using krypton 85 is just as sensitive, cheaper and more reliable.