An isolate of infectious bronchitis virus (IBV) was obtained from 7 1/2-week-old chickens showing acute respiratory distress that had been vaccinated twice against Massachusetts and Connecticut types of IBV. Little or insignificant neutralization of the virus, designated JMK, was obtained with the following IBV type antiserums: Iowa 609, Iowa 97, Connecticut, Massachusetts, Gray, and Holte. Antiserum from chickens recovered from JMK virus injection had a high neutralization index (NI) against JMK and Gray viruses, and a low, but significant (>2.0) index against the Beaudette embryo-killing IBV strain. Insignificant NI''s were obtained with JMK antiserum when tested against 2 Massachusetts-type isolates (Van Roekel and Connaught strains), Connecticut, Iowa 97, Iowa 609, and Holte IBV types. Vaccination of chickens in laboratory studies with Connecticut-type IBV resulted in insignificant protection against the JMK virus. No protection was apparent when Connaught IBV (Massachusetts type) was used. Better protection was obtained with the Van Roekel strain (Massachusetts type) of IBV, although it was still poor. The JMK isolate induced excellent protection against itself as well as against Massachusetts, Connecticut, and Gray IBV types.