Abstract
Over a period of several months, 5 germ-free mouse isolators became monocontaminated with seemingly the same fastidious, anaerobic bacilli which defied identification. The organism originated in rodent diet which had been exposed to γ-irradiation. However, it was discovered retrospectively that the irradiation doses had not been monitored, and subsequent monitoring showed the desired dose of 5 Mrad was not achieved in some parts of the load. Although inadequate sterilization was the likely cause of contamination it was not possible to explain why only this fastidious anaerobe survived the treatment, and not other more common bacterial contaminants.