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How to provide an adequate diet free from common murine pathogens is a problem facing the curators of all pathogen-free animal units. The commercial production of diets suitable for such units has not always been a complete success. There is the difficulty of obtaining complete sterility of the diet and the almost insurmountable obstacle of preventing contamination of the diet from the time it leaves the factory until it is placed before the animal. E X P E R I M E N T A L DietsFor the tests we used the following commercially produced diets for rats and mice: No.

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Airborne dust particles are an important vehicle for potentially pathogenic micro-organisms, which may thus spread from cage to cage. The dust itself may also be irritant to the respiratory tract, and produce a condition favouring the establishment of respiratory disease. A method has been devised of ventilating each cage in an animal room with a constant stream of air that is virtually free of particles, whether from another cage or from any other source. This method entails the use of a filter rack, which is simple and inexpensive, and so far appears to afford considerable protection. In effect, it places an air barrier round each cage. The complex aetiology of respiratory disease in the rat and the relevance of a filter rack to its control are discussed. The essential features of the design of a filter rack are given.

SUMMARYAn epidemic of acute respiratory disease in a colony of CFE rats is described, the main laboratory findings are recorded and its aetiology discussed.The epidemic showed that severe respiratory disease varying from peracute to chronic was associated with infection of the lungs withi a mycoplasma but that mycoplasmas could be present in rats, even in the lungs, without signs of disease, thus suggesting that one or more other factors were involved. It is also evident that there are strain differences in the susceptibility of rats to this disease.