A virus isolated from a respiratory disease in chickens was identified as infectious bronchitis virus (IBV). By serum neutralisation tests the virus was shown to be serologically distinct from the Massachusetts type but to possess a one-way relationship with the Connecticut type. Experimentally, it caused a relatively mild respiratory disease in baby chicks. Vaccination of young chicks with H-120 Massachusetts IBV vaccine afforded a high degree of protection against challenge with the isolate. Some problems relating to the isolation and classification of field strains of IBV are briefly discussed.