Two years later, Bushnell and Brandly (3) reported on an essentially identical disease. They established that the causative agent was a filterable virus because the disease could be transmitted by Berkefeld-filtered material. Because the symptoms of respiratory distress were caused by a filterable virus, Bushnell and Brandly regarded the disease as a form of infectious laryngotracheitis (LT). Further critical tests were not made to confirm this identity. Bushnell and Brandly's report (3) was considered by most later research workers in avian respiratory diseases to actually describe further outbreaks of infectious bronchitis (IB). It was not until much later that the possibility could be considered that some or all of their cases could really have been cases of the mild or lowvirulence type of LT described by Cover and Benton (4). A further source of confusion was that, in the 1930s, the name infectious bronchitis was used in some papers actually describing LT. This confusion was cleared up a few years later by Beach and Schalm (1), who proved by cross-immunity studies in chickens that IB virus was distinct from infectious LT virus and