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According to the histopathological changes of autopsy, certain kinds of catarrhal changes could frequently be found at restricted areas in lung tissues of conventional guinea pigs. When the essential changes caused by inhalation of sulfur dioxide (SO2) gas were considered, it was usually difficult to distinguish them from unknown conventionallyoccured changes. However, when the experiments were done using germfree guinea pigs, all data obtained from the experiments were quite free from such a disturbance. Sulfur dioxide gas of relatively low concentration was found to act on the lungs of germfree guinea pigs as follows: In the early stage, marked congestion and hemorrhage occurred in the lung tissues. In the later stage, diffuse or nodular thickening of the alveolar septa appeared markedly. Nodular changes were due to the proliferation of argentaffin fibers.

Increase of respiratory disorders among inhabitants and employees in industrial areas, such as chronic bronchitis, emphysema or asthma may be interpreted as manifestation of the combined actions of air pollutants. The constitutional nature of each subject, however, seem to associate closely with the individual susceptibility to the causes of such diseases, which may be provided in some cases by the interactions of pollutants, affecting previously, simultaneously, or subsequently. These may play important roles in some conditions to establishment of defence capacity of the respiratory organs of the subjects or, in contrast, as the causation of respiratory disorders. Potential threat of air pollutants to health may be expected not as much from any single pollutant as from pollutants, combined either with others or with air borne infectious microorganisms1)-4). Considering the role of microorganisms in this site of air pollution problems, new approaches to research are needed and the gnotobiotic techniques seem to be adaptable. Therefore, the studies were carried out with germfree animals to clarify the basic concepts of natural defence mechaisms of respiratory organs and interactions of bacteria and irritant gas interfering with animal health. Further, new techniques were developed in our laboratory for administering gases, dust or bacteria to animals reared in a germfree atmosphere. METHOD

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