Isolation of an unidentified agent from the respiratory tract of chickens.

A relatively stable filterable virus, not previously described, which hemagglutinated red blood cells of chickens, was isolated from the tracheae of birds suffering from an unusually severe outbreak of infectious laryngotracheitis. The latter disease concealed the infection caused by the new hemagglutinating agent, which was shown to cause, by itself, a mild and transient illness in susceptible chickens. Preliminary studies indicate that the new agent may be a new entity of the myxovirus group.