Vaccination and challenge experiments using infectious bronchitis virus (IBV) were conducted on groups of specific-pathogen-free chickens. Three weeks post-vaccination with one of the four IBV strains used, chickens were challenged with the homologous immunizing strain of IBV. Subsequently, the chickens were sacrificed, their tracheas were examined for ciliostasis, and the specific IBV antibody content of their sera was measured by enzyme-linked immunosorbent assay (ELISA). Results showed that protection was conferred by primary vaccination, as ciliostasis was not observed in tracheas from groups vaccinated and then challenged. No protection was observed in control groups that received only a challenge exposure, and the virus was readily recovered from their tracheas. Homologous protection was present in chickens that had ELISA titers as low as 624 and neutralization indices as low as 2.9, whereas susceptible controls had titers of less than 100 and less than 1.0, respectively.