A method is described of assessing cross-immunity in chickens afforded by strains of avian infectious bronchitis virus (IBV), by observing the presence or absence of ciliary activity in tracheal explants prepared from chickens following challenge. Chickens inoculated with the H120 vaccine strain were challenged intra-tracheally 3 weeks later with the homologous strain or one of seven heterologous strains of IBV. Vaccinated chickens were protected against challenge by the homologous strain (H120) and four other strains (Massachusetts M-41, Connecticut 46, Holte, Iowa 609), but not by one strain (T). The vaccine strain afforded incomplete protection against one further strain (Iowa 97) and less still against another (Gray). All unvaccinated birds were susceptible to challenge by all strains. The method is recommended for use in cross-immunity studies and for assessing strains for use in IBV vaccines.