Extensive testing of magnetic‐bubble circuits has been done where bubbles are propagated at frequencies up to 127 kHz and the long‐term retention of data examined. From tests performed on small circuits in a microscope environment, mean‐time‐to‐failure (MTTF) data have been taken. The effects on MTTF of garnet material parameters, ambient temperature, circuit design, ion‐implantation dosage, circuit defects and operating frequency have been examined. A high‐mobility material operated on a circuit modified as a result of these tests displayed near‐infinite MTTF over a temperature range of at least 28°C–80°C. Tests were also performed on large circuits in a memory‐module environment. The results of long‐term (∼1015 bubble steps) error‐free operations show that very large MTTF's are realizable in actual devices. A method for estimating reliability from these data is described and possible failure mechanisms for propagated bubbles are discussed in view of the new data.