Nuclei of a disease-free foundation stock of mice housed in a barrier unit and a barrier cabinet constructed from resin-bonded glass-fibre were compared over a period of 7 years. The cabinet was found to be a satisfactory alternative to a flexible-film isolator and was thought to be preferable. A description is given of the cabinet, the method of sterilization and the breeding schedules for 2 generations adopted to make the maximum use of the 10-month life of the high-efficiency filter employed. Throughout many cycles of this duration the degree of cleanliness was of a high order. A major contribution towards this was the collection of excreta on a tray lined with both absorbent and grease-proof paper, and the removal from the cabinet of dirt boxes when weaners were issued in them at the age of 5 weeks. These boxes were replaced in the cabinet by clean, sterile ones.