RESUMEN
Ten infectious bronchitis virus (IBV) isolates were recovered from broiler chickens in the states of Queretaro and Guanajuato in Mexico. The viruses were isolated from trachea, lung, kidney, and cecal tonsils of birds that showed respiratory signs in spite of vaccination with Massachusetts (Mass) and Connecticut strains of IBV. Each isolate was identified by an accession number from 1 to 10. Six of the isolates were neutralized by Mass monoclonal antibodies, whereas the other four were not. In addition, these four isolates did not produce lesions in embryos in the first five to seven passes. These four isolates were further characterized by the reverse transcriptase-polymerase chain reaction and restriction fragment length polymorphism techniques. The electrophoretic patterns for the four isolates were identical but were different from other known IBV isolates.