Abstract The term “air sac” infection has become associated with an exudative process occurring in the abdominal and thoracic air sacs of birds with chronic respiratory disease (CRD). It is this aspect of CRD infection which has been of the greatest concern to the commercial grower since it is the mortality and morbidity accompanying “air sac” manifestation that is of primary importance. “Air sac” infection can, however, be observed in a number of other diseases, such as infectious bronchitis, cholera, paratyphoid and Newcastle disease. The typical appearance of birds with “air sac” infection has been described by Wasserman et al. (1952), who suggested that the terms CRD and “air sac” infection are not synonymous since the latter is caused by a bacterial infection. Luginbuhl et al. (1954) observed that bacteria and fungi play a major complicating role in the pathogenesis of poultry respiratory diseases. This present paper reports briefly on the …