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SUMMARYFive strains of infectious bronchitis (IB) virus, which had been compared antigenically by the serum neutralisation (SN) test in tracheal organ cultures (OC), were arbitarily coded and then compared by the haemagglutination inhibition (HI) test. Their antigenic relationships were found to be similar by the two methods but, because of the high and variable cross reactions found in the HI test, the differences between the strains were less clear by that method. It was concluded that the HI test, in its present state of development, is considerably less type-specific than the SN test in OC, and cannot be recommended for defining antigenic relationships between strains of IB virus. However, it retains its value for diagnosing IB or for monitoring the vaccinal status of flocks. INTRODUCTIONThe lack of a reliable but simple type classification system for infectious bronchitis (IB) virus continues to give rise to practical problems and to confusion. Since the haemagglutination inhibition (HI) test for IB was first suggested by Bingham et al. (1975) and developed by Alexander et al. (1976) and Alexander and Chettle (1977), it has been used widely for monitoring serological responses to IB vaccination and for the diagnosis of IB infections. Lashgari and Newman (1984) used it to compare seven reference strains of IB virus and found that although they were antigenically distinct, complex relationships existed between them. Brown and Bracewell (1985) found that the HI test could be used to type IB virus strains, provided that the test was carried out under carefully controlled conditions and that all virus strains were tested against antiserum prepared against each strain.

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